And of course when Michael said "took it [the ring] off a nigga" Marlo would've assumed he'd taken it off Omar.
Respect.
i think the thing with the ring does tell us a lot about how property is seen in west baltimore - like omar says "money don't have no owners, only spenders" - this applies to any property - the only owner it has is the person who can take it, and not have it taken off of him.
Even if Marlo didn't assume it was omar that micheal took it off, he knows that micheal has at least taken it from someone - therefore he's a predator in the game, not the prey, and not someone who will be at the bottom of the food chain. it's what marks someone out, as deserving of some kind of respect.
there are also parallels here with bub and his shopping cart - that guy just takes his stuff, because bubs is weak, therefore doesn't get respect. officer walker also takes stuff from bubs - because he knows there wont be a comeback.
also seen where kenard steals namond's package - he senses that namond is weak, so tests him out. and also in marlo taking those lollipops - the guard knows he has got to step to, otherwise marlo would be there everyday taking. same applies to corners, with marlo taking the corner that bodie was working...he didn't want it when it was shit, but once it became valuable, he was having it. The ability to take, without fear of consequences - that's where the power is.
like namond etc say in the classroom - everybody always be watching. seeing waht you do, seeing if you are weak, and if you are weak, they'll come in and take it from you.
the simple fact of micheal having the ring shows marlo everything that he needs to know about what kind of person micheal is, and whether he'd be right for his organisation.