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!