Nemean and Thorns were slightly nerfed. They're still good, but Thorns isn't a must by. If your team needs a utility relic second then skip Nemean and get Midgardian, which is really good right now.
This guide is written from the perspective of a Masters player, and is useful for both ranked and casual conquest. My goal is to provide readers with a more nuanced look at the character, compared to other guides. While the content is more directed towards intermediate players looking to transition into higher level play, beginner and advanced players will find this guide useful as well.
With the exception of items, most of this information should age well. As a precaution I will generally avoid mentioning numbers when it is not necessary.
As a side note, the item builds also work with other auto attack warriors such as Bellona and Erlang Shen, as their roles and functions have historically been very similar.
In this section I will go over how to maximize the functionality of each of Osiris's abilities as well as his passive. Playing on instant casting (except for the ultimate) improves the fluidity and the accuracy of the character.
Fragmented - Grants Osiris physical and magical mitigation, as well as the ability to pass through players and player made obstacles, and ignore movement speed penalties for a limited amount of auto attacks when attacking at 8 stacks. You want this at 5 or 6 stacks before the waves meet and before team fights as it greatly improves your survivability at all points of the game.
Sickle Strike - Use this at level 1 in the well to stack your passive before the waves meet; if you're in a hurry you can also spam it on the way to lane as the mana cost is very low. While it is mostly preferential, you should max this first if you're against characters that want to box you, such as Bellona, or characters with low mobility. It is also a very good auto attack reset.
Spirit Flail - You generally want to use this after your sickle to increase the slow magnitude and reset the duration. There's a few reasons why you would want to max this first. When jungling you want to quickly clear camps, but also in the solo lane you may find that your autos are not enough if your opponent is using their clear to out-trade you via minion advantage, in which case this is more useful for trading than your sickle. Additionally, if you're going against characters that you will have a hard time sticking to, like Sun Wukong it is better to max this and use for the poke. The movement speed helps for escapes or chases as well. This is overall better to max first, especially with the tankiness of minions in Season 5.
Judgement Tether - Hit as many people as possible with this in fights to improve the longevity of you and your teammates. In the laning phase, the damage reduction can hinder the clear of your opponent. For example, an ability that barely full clears will no longer full clear when this debuff is applied, forcing your opponent to be near you longer or use additional mana. Max this third if you respect the damage your opponents put out, particularly the mid laner. This is also one of the strongest auto attack resets in the game.
Lord of the Afterlife - This can be used as an escape or initiation, or mid fight for the targeted anti-heal or to reapply your passive. Use after your tether stun or CC from your team for a guaranteed hit as the circle is rather small. The mini root may be useful to set up skillshots for your team if you have communication. You may choose to level this over your tether if you're ahead, the damage is on the higher end of warrior ultimates.
In this section I will explain good and bad itemization options available to you. I will only use completed items but may mention their components. Against a standard 3 physical, 2 magical team, a general rule of thumb is to have around 3000 health and 200 of each defense. Also, it's a pretty good idea to select your first two items after boots towards the enemy solo/jungle as that is usually how long the laning phase will last.
Jungle and Solo Osiris basically serve the same purpose in games and their itemization does not change much. The only difference is that jungle Osiris may be built more hybrid without worrying about losing effectiveness.
For the sake of this guide, completed boots will be considered the first item.
Core Items - these items should be built almost every game.
Chalice of Healing - Essentially provides you with an additional 750 health, useful at all stages of the game, should not be sold.
Warrior Tabi - This is your strongest power spike, if you get an early kill and buy this before your opponent does, it drastically improves your snowball potential. Ninja Tabi should never be bought due to Osiris's naturally fast attack chain and reliance on auto attack resets. Sell late game for Reinforced Greaves when your damage no longer matters.
Ward- self explanatory.
Situational Items - these items vary depending on the state of the current match.
Health Potion - Starting 2-4 health potions instead of boots will let you smoothen your trading and farm in boxing heavy matchups. Not necessary against most guardians.
Warrior's Blessing - Probably the best starter for Solo Lane, Osiris can proc it easily due to his ranged abilities.
Hunter's Blessing - Maximizes your cleave auto attacks, allowing Osiris to clear better and bully more.
Gladiator's Shield - Better in S5 now due to how long the laning phase is for solo. Osiris trades very well with this.
Void Shield - The S5 changes made it more accessible to the solo lane. Can be useful on Osiris as it benefits his aggressive playstyle in lane. The health is fairly low so you shouldn't build it early if they have a magical jungler. It's still good to build it late once you have other defenses online. Also a good replacement for gladiator shield late game.
Mystical Mail - Better fulfills the same purpose as gladiator shield, however, the health enables it to be useful even if they have a magical jungler. It's pretty expensive and can be hard to itemize into in the rare cases that Osiris falls behind. It also allows you to save your flail solely for fighting. You can build this after gladiator shield but should not vice versa. You should not build this if you build void shield as it delays your magical defense too much. Not as good now in Season 5 as the damage it does falls off due to the extended laning phase for solo, and mages building Warlock's almost every game now.
Stone of Gaia - This is a particularly useful laning item when built second against guardians, as their low damage gets countered by the sustain from the passive and chalice. Also, most solo guardians can usually proc the knockup aspect of the passive as well, increasing it's value. Build this third and you will be extremely difficult to kill during the laning phase. Sell this late game for a different health item if they have a lot of antiheal.
Pestilence - The anti-heal is useful almost every game due to lifesteal and other forms of itemized healing, it's a cheap fourth slot magical defense item with great stats, and most supports go other magical defense auras.
Hide of the Urchin - This item is probably underrated right now. The S5 changes reduced the protections but increased the effective health when fully stacked. The only problem is fully stacking it, making it a little risky.
Runic Shield - A cheap magical defense item that aids you in diving their magicals, as well as increasing the survivability of your team. If their mid is fed then build this fourth, if you play your cards right the added power will help you shut them down better than pestilence.
Ancile - Counters certain guardian solos. Build this as a second item against Cerberus to cancel his breath attack. Can be useful against Artio to stop her life tap, although difficult to pull off. Makes it difficult for Ymir to combo you if he only has one point in his freeze. Cancels Cabrakan tremors. The power and cooldown is also a nice bonus.
Mantle of Discord - Gives you a lot of protections to round out your defenses, the passive can be surprisingly useful against almost every character. The cooldown and magical defense make it a logical upgrade to gladiator shield, but it can be picked up in guardian solo match ups third after stone of gaia, as it is cheaper than buying two seperate defense items, making you very tanky against both the solo and physical jungler.
Hide of the Nemean Lion - Deals damage before mitigations, countering the lifesteal from devo's and forces hunters to build a second lifesteal item, reducing their damage. If they don't build double lifesteal then you simply kill them. If they do, then nemean + thorns is your best way of preventing them from impacting the fight. Can be built instead of glad shield in the second item slot, it makes you very tanky.
Midgardian Mail - Serves a similar role as nemean, except the health is useful against magical and true damage. In cases where they build a lot of penetration, or you had to pick up a second relic other than thorns, this increases your survivability more than nemean. The slow also makes it more effective at countering non hunters.
Masamune - More of a jungler item as they can get away with building hybrid more and the movement speed is good for jungling as well. However if you're far ahead, this isn't a bad idea, the health is useful and the potential 35 of each protection can make this a good defense item.
Stone Cutting Sword - Sell boots for this late game if you built masamune, you essentially have the same movement speed as a regular game with boots, but 60 additional power and the passives.
Winged Blade - Replace stone of gaia with this if they have a Nemesis, or if you've died to the same slow one or more times before starting stone.
Spirit Robe - Can help you survive against a CC heavy comp. Disarms and silences can cause this to proc. sooner than you might like however. Can be built early or as a substitute to mantle.
You can also build aura items that your support hasn't/won't build this game.
Sovereignty is particularly good, especially as a replacement to mystical mail in the jungle if you're going a more utility approach. The timing for it is difficult in solo as if you replace mystical with it you won't be getting the full use out of it until later in the match.
The only support aura that you should not pick up is Shogun's Kusari, because the base stats are quite awful and building it would not help you as a frontliner.
Teleport Glyph - This is probably the biggest winner in S5. The bigger map and the changes to minion aggro priority makes this all around better. Back and teleport as soon as you have enough for boots.
Horrific Emblem - Greatly increases your level 1 kill potential. Make sure to tank an attack before popping to draw creep aggro onto your opponent. Minions do less damage now but any DPS increase helps.
Shield of Thorns - This is a strong second active as you transition into the team fighting phase, but shouldn't be picked up first because you'll rarely get the full damage out of it when fighting the enemy solo/jungle. Pairs well with Hide of the Nemean Lion to shut down hunters.
Heavenly Wings - Choose this over teleport if you feel like you might die to an enemy horrific or Nemesis ultimate, or if you're going for a riskier start such as proxying the wave or starting at the enemy jungle camp. If your team doesn't have one yet it is more useful to get than thorns.
Magic Shell - Protects you and your team from dying or can be used to bait if you have communication. The upgrade is useful for your team but counter-intuitive with nemean.
Meditation Cloak - Preferential alternative to shell; while the damage protection is lower it can be used reactively making it easier to utilize. The mana sustain may also be helpful.
Purification Beads - Mainly picked up on jungle to offset less defense, however may be situationally useful if they have CC that would be hard for you to avoid or live through, like an Awilix paired with a knockup heavy team comp.
Aegis Amulet - Similar to beads, better on jungle and allows you to bait abilities and outplay more effectively than teamfighting actives in certain situations.
There you have it, now you know how to play and build Osiris better, hopefully. I'll try to keep this guide up to date, and maybe add in more information if you guys ask for it. If you have any questions about the guide or god itself feel free to comment and I'll do my best to answer.
1/28/2018- Published guide
1/28/2018- Added actives section
2/6/2018- Updated for 5.1
2/19/2018- Updated for 5.2
2/24/2018- Updated for 5.3
3/19/2018- Updated for 5.4
4/16/2018- Updated for 5.6