"oh sure" is another way to disregard what someone else has just said, or to express that he doesn´t believe something just said (even) when other do.
Of course, it could mean both, that you are agreeing with something or that you don´t agree with what was stated.
An example of an ironical way, with "oh sure":
She glanced down at the towel tucked around his midsection and gasped.
He laughed. "You've been admiring my chest, now let me admire yours." He reached for her robe.
She slapped his hand and clutched her robe closer. "I don't think so." She started for the patio door and he gripped her arm.
"Oh, come on now. I'm only joking."
"Oh sure, and if I started taking my clothes off, you'd tell me to stop, right?"
It happens the same way with "oh sure" without complementary information, just "Oh, sure!" (it could express that you don´t agree, trust or it´s mild annoyed, or all of them at the same time...)
But, be aware of the context: it goes with the intonation (what is called intonational collocation).