I always use "ok" when I talk to my coworkers and want them to know that I listen and understand what they say. I found that it's redundant and repetitive and want to know alternative words that mean the same thing.
If you're speaking face-to-face, a simple attentive look towards the speaker is sufficient-- you don't have to stare, either. Overdoing it with "okays" and "yeahs" is pretty annoying, in my opinion. When I listen to someone speak, I don't say anything unless they're saying something I actually want to agree with or re-affirm. At that point, I usually say one of the following:
The other case I would speak is when I have something to correct them on, or disagree with. I am of the view that your actual response to their statements provides better evidence that you are paying close attention than robotically saying "hmm" or "yes" after every other sentence. Hearing other people do that annoys me because I can no longer tell if they are actually listening or just pretending to listen.