Be careful about using circular interpolation to mill an "O" ring groove. Had bad doing that on high pressure hydraulic valve bodies.
The pattern left by the end mill (63 rms) allowed a minute amount of hydraulic oil to leak. The leak eroded the "O" ring and it failed. This happened in a matter of minutes.
Solution was a hollow mill type of tool that left a pattern which laid in the same circle as the "O" ring.
I don't know if this applies in your case, just a "heads up".