Who was more evil -- Nino Brown or O-Dawg?

That's a great question. I wouldn't necessarily use the word "evil" to describe O-Dog though.

Nino Brown, in my opinion, was a narcissistic sociopath the reason being, Nino acted out of absolute self interest at the exclusion of anybody else. Some examples:

Holding the "civilians" under siege at The Carter for his drug operation
Using a child as a shield during the assassination attempt
Sleeping with Uniqua
Killing Scotty's mom in cold blood
Turning on the other members of CMB during his trial

In all those cases, Nino's first and only instinct was to protect or please himself regardless of whom else he impacted. I think it's brilliantly contrasted with the recurring theme: "Am I my brother's keeper?" In his words, Nino tried to portray himself noble but the reality was that he'd step on anyone no matter how close they were to advance himself. And without conscience no less. You're right: he was absolutely cold-blooded.

O-Dog, on the other hand, actually seemed to care about the people in his circle. I haven't seen Menace in a long time but despite how desensitized he became to violence, it was always directed at "outsiders".

O-Dog arguably operated by a warped code of honor (that Nino didn't have) that grew out of the violent environment he came up in. I don't recall him having a blood family in the movie so his family became the people he came up with. The only guidance he had was how to survive in his world. So the people who would perceive him as "evil" were the "them" in his us vs. them mentality. To his friends, he was a person who would do anything for the handful people that meant something to him.

Nino hands down. He killed his best friend, used a kid as a bullet sheild, and lit a guy on fire for $10 (or something like that)

O-dawg was every bit as cold blooded as Nino! The big difference was that O-Dawg was content to only inflict his evil on any poor bastards who happened to cross his sociopath, whereas Nino was a blend of smart, ambitious, and evil.

There both evil, but different ways of evil if you will. The edge is to Nino though.

Nino is shown to have little care for anyone BUT pretends he does. He had no remorse for things he did.

O-dog obviously was raised in chaos and lives that way. His one redeeming line when speaking to Caines Grandpa - "shit... I mean things are messed out there." so he seems to know what he does is wrong, but has long stopped caring.