Oil cooler is driver's side. If your engine is in upside down, that is!
Anyway, my guess is your lower head bolts are seeping where they pass through the head. They need some kind of sealant where the flat washers and nuts go inside the rocker box, or they'll seep oil.
I can't think of anything else that it could be. Leaking oil can't pass from one fin to the next, without flowing uphill in between, so it can't be a case bolt or cylinder spigot at the crankcase end. The other possibility was pushrod tubes, but they look clean.
If it is your lower head bolts, you can probably remove the nuts one at a time, clean the seat in the head, as well as the washer and bolt, and retorque with sealant. I think I used flange sealant on the washer (both sides) and thread sealant on the nut. You can get to the nuts with the valve cover off.
One of our experts should comment on this approach, as the stress on the head is uneven while you remove the nut, but before you've retorqued it. If it were my engine, I'd try it though. As long as everything has cooled down to room temp.
Jim