Is it OK to tell someone who is offering a job, that I actively have another offer in play?

Submitted by Anonymous (not verified)
in
Career Management
Will I essentially kill my existing offer because I want to actually here another equally compelling potential offer coming. Is it OK to say, give me a week to decide? Decision is about scope and money. I am wary about telling people this, but at the same time it's my career and I think I should not be playing first come first serve. Thoughts anyone?