Weight of oxygen

Yes, by a little bit. Air is mostly Nitrogen with some Oxygen and Argon mixed in. Nitrogen weighs slightly less than Oxygen.

(Who is the jerk who thumbed Engineer's perfectly reasonable response down?)

It would be more correct to make this a question about density rather than weight. The difference is important and "weight" doesn't exactly make sense when comparing gases.

Nitrogen is slightly less dense than Oxygen.