An o-ring might not work as well as a flat garden hose washer.  The thicker cross section of an oring would need to be tightened a bit more, thus squeezing the oring in place.  Rubber tends to take a "compression set"  meaning if you squeeze it too much, it will relax and deform over time.  I originally suggested the flat section hose washer to prevent the compression set.  I've used the hose washer for 6 months with no issues.  The engineer in me thinks the oring may degrade in performance over time.   Pehaps, perhaps not.  Time will tell.