Pinch it off before it has a chance to flower by cutting the central stem just immediately above a pair of leaves. I harvest mine with a pair of kitchen shears at least once a week - once they flower, they lose a lot of flavor, I think. You can be fairly merciless, but don't cut more than half the stem's height at one time, or you will over-stress the plant.
If you can't use all that oregano at once, freeze it. I put mine in small cube storage containers with a little olive oil. They go into the microwave to thaw, and right into the crock for sauce. (Same goes for basil, too.)
-=>epm