So, I've done all the College students' quests after which they became my friends and I could have them as followers. In Onmund's dialogue, I had an option to take him with me. I didn't the first time, I took J'Zargo instead. Then he just stopped following me; I think I accidently killed him in a battle somewhere, but never saw his body. So I went back to the College to take Onmund with me - only to notice he doesn't have a "Follow me." dialogue option anymore. Brelyna, on the other hand, still has. What's with Onmund?

Are you at a certain point in the college quest line where you've got a big job to do? There was a point where I couldn't interact with anyone (vendors, trainers etc.) until I finished my open quest.

Nope, I've only just spoken to Augur of Dunlain and told his/its information to Arch-Mage Savos Aren. You can also spoil for my part since I've played throught the College MQ before. I can normally interact with everybody else in the College except Onmund's "Follow Me." option is missing. Even Onmund's dialogue works normally otherwise.

His greeting line with me usually is "You're nothing but trouble", though. Is it random, or has he become angry to me or something? I haven't been (caught) stealing or otherwise violated him or College rules...

I think it's just a bug that the dialogue option isn't available. Does anyone else know about this? Any way around it? I'd like to have him with me since I'm still pretending to be a good guy, just an ordinary wizard student. >: )