There's an o-ring that fits on the brass powertube tip to make sure it seals when you screw it down. Then there's an o-ring that sits behind the brass spacer inside the powertube. Those should be the only o-rings you have.
If it's still leaking after you checked both of those o-rings for wear and nicks, inside and out, then you might need to adjust your spacer. I believe one of the more common sizes is .220, but they vary in increments of .005. If you only have the one spacer, hit Tunaman up for a spacer kit.