Should I suspect the "O" ring behind the clutch? 65K miles, I baby the clutch from a stop, and rev match gear changes. Maybe a little oil drip found its way onto the friction plate?

That is possible but more likely the clutch just needs adjusting I think.
I have seen lots of oil in the bellhousing from a leak and none on the clutch,
my thinking is that it only leaks when the engine is running which means the clutch is spinning
tending to throw it off.
 Possible, as I said but I would expect lots of oil to be dripping from the bellhousing.

If you suspect crankshaft seal park on main stand makes it drain off quick so no accumulation inside. I agree about the comment on oil being spun off too.

I have a manual, but what is the quick and easy clutch adjust procedure

Thx IK. I will take a look at those adjustments  ASAP

I have approx. 1 cm of free play at the clutch release lever

Way too much free play.  It should be 3-4mm, about 1/4 of the amount you have now.

Is your clutch feeling light and smooth? A frayed or sticky cable or sticking somewhere could cause slippage and result in excess free play but clutch action would feel stiff. Check the cable and adjustment (and actuator) before going at the clutch itself. My cable frayed in an unseen location and left me with a lot of free play because it was preventing the clutch seating properly. Change of cable cured it all.

@ARTCOLLINSJR wrote:I have approx. 1 cm of free play at the clutch release lever

Check the clutch arm at the back of the gearbox. If the bearings in that are seized, then the arm will not return fully, giving you a lot of play at the handlebar lever, and possible clutch slippage.

That's a needle bearing, replaced mine same time as the cable. It was bad. That going stiff had caused excess pressure on the cable which accelerated the wear and fraying, along with it being incorrectly routed when it was installed by a previous owner.