An exclamation that expresses both surprise and pleasure; a combination of the phrases "Oh my God" and "Nice!". The phrase "Oh my tart!" can be used as a substitute, with tart meaning tight (in the sense of cool or sweet).
Person 1: Hey, we have some left over pizza, do you want to get in on this?
Derived from Red Dwarf, it is used when Rimmer attempts to simulate Jacobean courting. It means a man who uses a cheesy move to try and pull a woman. It's very similar to "Oh Matron!" from the Carry On films.
Katie just said that I had a lovely smile. She's hotter than the sun! I'll try yawning and putting my arm around her slender shoulders. Oh my lady!