Fenrir Jungle

There are a few technicalities here to consider.

• Magical protection: Bulwark is 20 higher (80 vs 60 for Oni)
• Health: Bulwark is higher (250 vs 100 for Oni)
• MP5: Oni has it, Bulwark doesn't.
• CCR: Both have 20%

As for the passives, we would need to dive deeper. Though I'm not going to go super deep into the math, I'll make some general assumptions.

Fenrir has base health of 2,015 at level 20. Add in Bumba's and it's 2,315. Add Oni and it's 2,415, or Bulwark and it's 2,565. On the protection side, he'll have a base 48 magical protection. If we follow the rest of your build, keeping Prid but replacing Mantle, he'll have 138 prot w/ Oni and 158 w/ Bulwark. Of course, we assume that the enemy will likely have flat / % pen to reduce this to a degree.

Now, sometimes we underestimate mitigations, as they reduce damage from all sources (including true), and they're not subject to reduction due to pen like regular prots are. The increased health from Bulwark base and passive are subject to items like Qin's Sais, Heartseeker, Soul Reaver, etc. as well.

That being said, you're going to have enough protections that outside of true damage abilities, you're going to be mitigating a decent amount of incoming damage already. If they even get his magical prot down to ~70 (which w/ Oni would require 40% pen + 15 flat), he's still going to be mitigating 40% of incoming magical damage. If we apply the same pen with the higher prots from Bulwark, the mitigations increase by ~4.5% (roughly half the extra mitigations if we assume full Oni passive use). I would say the extra base health + the 385 added health from Bulwark easily balances out the Oni passive, at least on the magical side.

On the physical side, Bulwark only has the added health going for it, so in some ways Oni may be a bit more functional in countering physical damage, though not by a huge amount.

Oni not having a cooldown is a good point, but the requirement for more enemies to be close to get the full effect should also be considered.

In the end with these considerations, I think in most respects the two items end up being pretty similar in protection function in this type of setup. The only other point I'd mention here is that the rest of the build has no MP5. Oni's is pretty nice, so the mana sustain there means less need to back if you're using abilities regularly (which you will since you're maxing CDR), and that one respect would make me consider Oni more strongly.

But still...this build setup has major issues to start.

• Jotunn's really isn't a bad item for him, and I'm not sure you need to sell it late. It'll form the base of his CDR needs early, with Bumba's Hammer upgrade maxing him late. I really think, if you do want Bumba's Hammer as the upgrade and Jotunn's early, you should be completely dropping Pridwen and Mantle of Discord from the build example. And why?
  
  
• You're lacking power and pen. Fenrir in a jungle role really should be working in more damage, especially since he's got really good scaling on his abilities. 3 items with no power or pen in your build example really kills his damage. There are many options you can consider as alternatives to Prid and Mantle, and even Void. You could go mostly full squishy by getting things like Transcendence, Hydra's Lament, Heartseeker, Serrated Edge, Titan's Bane, etc. Instead of getting Bumba's Hammer if you're overcapping, you can go Bumba's Spear instead.
  
  
• That said, tanky Fen is also fine, and might be more functional if the enemy team is exceptionally bursty or maybe your solo went off-meta squishy themselves and you need another front-liner. In that case, you could still consider Pridwen in combination with Jotunn's Wrath, and get Bumba's Spear or build more into something like Eye Of The Jungle / Protector Of The Jungle instead.

But overall, you really do need to get away from that crazy CDR overcap. That would be my first suggestion, and then reassess your item options from there.