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.
-Aha (with lips either open or shut!)
Downvotes have made me think over this question and I found that you want words that show that I am listening and understanding (and NOT agreeing) what you're saying.
There are many ways. A few of them are here-
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.
There are many ways in responding to the speaker's message, verbally and non-verbally. You could say other words such as "yes", "all right", and "sure" being some of them. Nodding is the commonly used response and one way of responding to someone talk.
Same issue as with Ruchir M's answer - your question was about something else. Due to this, I've updated my answer:
until now all fine