I don't think I've further clever insight to offer on the subject of Omar's death, except that unless I remember totally wrong, it's worth noting that Omar is in the end killed by Kenard, who was one of the kids playing on the street after the big shootout in Season 3 (the failed stash house raid, where one of Omar's crew, Tosha, gets killed). Kenard is the little kid who's playing the part of Omar, as it's clear that the neighbourhood kids idolize Omar (which is referred in that brilliant dialogue between Omar and Bunk later on in the season). Then few seasons later when Kenard is in the game, he sees Omar limping along (after being crippled by the fall after a shootout with Marlo's crew) and no doubt sees himself an opportunity to become a player himself. Very Jesse James/Robert Ford-ish.

Funnily enough, the real-life Omar, Donnie Andrews actually got out of the game and is doing well these days. And as a somewhat OT note, to me Frank Sobotka was easily the most tragic and complex character in the series (actually, I loved all of the dock gang). Chris Bauer did an amazing performance as him. I can't call myself a blue-collar worker, but the characters at the docks were written so well, it was real easy to identify with alot of those issues, even if my background in physical labour is mostly limited to a few summers working in construction and road work. S2 was some heartbreaking stuff (even if S4 is the best season overall). One of my favorite montages from the series, still kinda difficult to watch.
http://www.youtube.com/watch?v=SMe-3jFdwLY Frank Sobotka: We used to make shit in this country, build shit. Now we just put our hand in the next guy's pocket.