If your o-ring is only squared and not torn, wrap some plumber's Teflon tape into the o-ring groove (under the o-ring, important.) This will close the gap--it doesn't take much tape--and the pump will work again. Grease with silicone spray or bike grease. Both work. Silicone spray is cleaner.

My airtool&o-ring is from '99 and still going to 100#, although the top part of the cylinder has become a bit leaky.