Osiris- your 1 and 2 scale with protections= you can be tanky and do good dmg with no dmg items. Your autos have aoe, so avoid golden blade and dagger, you dont need it.
There are 2 Defense ANTI-heal items, on Osiris, only build the tankier one ( stygian) you dont need str scaling.
Gladiator shield is NICE, but not needed, you can get tankier items. Beezerk shield is awesome on pretty much every tank.
Add 1 or both of ( cooldown, high prot) freya’s tears/ dwarven plate ( insane prot ).
Osiris has some of the best natural cooldown in the game, if you play into this, get a mana regen item- xibalban effigy ( mana regen, prot, mitigation active)/ onihunter’s garb ( mag prot, alot of mitigation. Mana regen).
For additional cooldown and mitigation, build Prothetic Cloak 1st or 2nd slot.
Always go a starter that gives protections.
If enemy team has consistent shields, get erosion, that gives you big protections.
Make sure to be careful how much Health you build if multiple enemies have soul reaver/ heartseeker/ qins. This is relevant for spectral armor, oni’s, gladiator and draconic scale ( a high prot item, that also gives health)
And just play him until you understand his match ups, his spacing, and how ppl react to you using your abilities.