The last game I've played was exactly that. My first observer finishes at 6:45, at 7:00 he's at the front of my base with 16 marines. (cut scv's at 15, 5 barracks without addons) In fairness I could have held it off with a 3rd gateway and a better unit composition.
edit: tried the expand thing. I think I can have enough to hold most cheese off and get an observer out early. tyty