OB

Ob is short for 'OBviously'.

Person A: "You want to go get some of that pu-tang?"
You: "OB!"

act

Act is short for ACTually. It is only used in the indirect questionative form, meaning it is only used to question something someone says but not really looking for an answer.

Person A: Yo that fly hunnies diggin` yo phat ride!
You: ACT?

Just like saying "Really?".