This is my guide on Osiris. I have played solo for a long time and personally enjoy Osiris very much. Keep in mind this build is for solo exclusively.
My build is split into 3 sections:
This is what you should build at the start of almost every game. Warriors blessing is crazy right now and every solo(including mage or assassin solos)should pick this up. The health chalice and health potions are for the maximum sustain. This setup will allow you to stay in lane for a long time. As long as your jungler plays like a functional human you should get your mana buff. If you are worried your jungle wont get your buff feel free to pick up multi potions.
This is what I would recommend building against certain types of solos.
The phys-ability build is optimized for early pressure against characters like Sun Wukong, Hercules, or Amaterasu. When building this, make sure to be careful when you have tier 2 of mystical mail, as this is your weakest point before late game. Once you have mystical mail play as aggressive as you can. Unless you are behind you should be able to box any enemy.
The phys-AA build is best against characters like Bellona, other Osiris', or Erlang
Shen. Once you have Nemean Lion, you are essentially unkillable unless you play poorly. This is your strongest stage all game. Mystical Mail gives you insane box. Osiris plays very well into other AA based characters, as his boxing potential is arguably the best of any warrior.
The magical build is very powerful when you are fighting characters like Artio, Cerberus, or Cabraken. The pestilence hard counters all characters with sustain. Although mystical may seem like an odd pickup, the physical protection will help with the jungler(If there is a magical jungler it would be best to build an item like shoguns kusari or bulwark of hope) and the HP and passive are still effective agaisnt your opponent. Osiris should win against almost every magical solo laner.
The late game
These items are all optional items after your first 3. Obviously you should sell warriors blessing late game. Any of these items can be effective in the right situation. I'm not going to go over them all, but if the enemy team's midlane is doing a lot, you should pick up a bulwark of hope. If they have a slow heavy team Winged Blade is a good option. Remember that your job as a solo laner, and more specifically as Osiris, is to attack the back line. You want to be as tanky as possible so you can fight for as long as possible.
Before I get into the leveling order, first let me explain each of Osiris' abilities.
This is a line ability that stops on the first enemy hit and slows them. The sickle remains for 3 seconds. Although at first glance this ability is weak, it has a cool down of only 5 seconds. At level one, this is almost a permanent slow. Because of this ability, Osiris' level one is arguably the strongest of any warrior.
This is a circle ability that does damage and increases Osiris's movement speed. If the sickle from sickle strike is still in the enemy, the slow is magnified. This ability is good to combo with sickle strike to keep an opponent slowed for a long time.
This is a big circle around Osiris that tethers enemies to Osiris for 4 seconds. While they are tethered they deal reduced damage. At the end of the tether all enemies still tethered are stunned for 1.4 seconds. This ability should be used when as many enemies are close to you as possible. The stun at the end can be used to set up your sickle or your ult.
Lord of the Afterlife
This is a leap that renders Osiris CC immune and roots all enemies caught in the radius. In addition, all enemies in the radius have 100% reduced healing. This ability is an effective escape and engage tool. The healing reduction is massive and if you hit more than 2 people with this your team will likely win the fight.
Every time you cast an ability Osiris sheds a shard of his soul, granting him damage mitigation. When Osiris has shed all 8 fragments of his soul he becomes a ghost for his next 8 successful auto attacks. Osiris can pass through player made structures and players, and has haste. At level one, its best to cast your sickle 4 times and go to lane. Why 4? Because you should cast your sickle almost nonstop at your opponent once you are in lane. By casting it 4 instead of 8 times you get to lane 20 seconds faster.
My leveling order makes Osiris into as big of a lane bully as possible. Although it does less damage, the sickle has half the cool down that the flail has. Always max your sickle in solo lane. I max the flail next as it grants Osiris better boxing. The ult is risky to use, as if you get ganked you will likely die if you don't have the ult. I would only use the ult offensively to either secure a kill or if you are very confident in your ability to not die.
Osiris is an auto attack based warrior. His chain is .5 1 .5 1, with his last two AAs doing cleave damage. This chain is very fast and fits Osiris' aggressive style. In lane you want to play aggressive, casting your abilities as frequently as possible. Because of warriors blessing, Osiris can mostly ignore minions when boxing. You should try to focus the enemy, trying to hit them with every cast of your sickle. When you are ghost form, you should play very, very aggressive. You have huge damage mitigation and cannot be body blocked. Whenever your tether is active you should also play aggressive. When they are stunned be sure to hit them with a sickle or even the ult if you are feeling spicy.
Late game is different. Osiris is very tanky but doesn't have a huge team fight presence outside of the ult. What you should do is constantly attack the hunter or midlane and prevent them from damaging anyone but you. At this point, you should dive until you are about 25% HP and leap away and disengage.
On of the most interesting things about Osiris is his combos. Because of the way his sickle works with his autos, an excellent combo is as follows(A means auto attack)
This optimizes the damage dine, as both of these abilities quickly reset the auto attack chain. Alternatively, you can use
Remember to cancel your auto attacks as often as possible with your sickle
I hope my guide has helped you play Osiris. Osiris is one of those characters that will almost always be strong. His lane bully is so strong and he scales reasonably well into late game. Enjoy playing!