Well, actually the diameter is a bit larger, but yes. The o-ring just gives it something to compress it into position as well as seal it. Next time you service yours, take a look at the inside. I just simply rolled it on to the base.
When I first installed mine, I didn't use this O-ring and not going to lie, the gurgling was absolutely frustrating and annoying and all these bubbles kept coming out of my spray bar. I finally figured out what it was, the water short-circuiting and not dissolving all the gas and it was all sneaking out the riser. Now, I would almost say its 100% dissolved inside the reactor.