Depending on how you say it, "Yeah, OK" can have several different meanings:
- It can mean that you don't believe what someone said.
He told me that he was a good dancer, and I was like "Yeah, OK." But it turns out he really is good.
- You can say this when you accept someone's offer.
A: Want to go grab lunch?
B: Yeah, OK.
- It can mean that you've understood what someone was saying and now you're annoyed with them.
A: Don't forget to call me when you get there.
B: Yeah, OK, mom. I've got it!