I took my O2 Sensor out this was weekend to clean it, (following instructions posted on this link). My motor is a 1999 OX66 225. When I removed the barbell piece, there was no O-ring or metal O ring as described in the "how to fix it link" on this site. Also, when looking at the parts diagram from SIMYamaha, these two items are not listed either. I bought new gaskets, cleaned everything and I am ready to re-assemble. I just wanted someone to confirm that these two O-rings are not used on this year motor and HP. If they are supposed to be there, someone please give me the part numbers so I can order them.
I might add, I had very little carbon build up and my motor was last tuned up 2.5 years ago. At that time the mechanic told me I had a lot of carbon build-up. 2.5 years ago I changed plugs to an 8 from a 9 and changed oil from Yamalube. I now use Pen. Seems like it made a difference. For the record, I use the boat at least 38 weekends a year.