Napa/Wix fuel filter?
 

Changed new fuel filter, 2001 F250, 7.3L diesel, on top of engine a few weeks back. Just found a "O" ring in box. Does this "O" ring go on bottom of filter? Have had no problems and runs great. Thanx. MTG

You might have lucked out and reused the o-ring that came with the filter you replaced.

Yes it does.

I ran mine without that o-ring for 6 months. It ran, do not know if it let anything through to the injectors though. I am running the Racor pre-pump strainer as well though.