If you managed to block off all the air flow through the cooler, you would still get significant cooling effect.
A thermostat is a good place to start. But, even with a thermostat, you'll still get some cooling. The thermostat allows about 10% of the
oil to flow through the cooler when "closed". That allows all of the
oil to warm up at the same time. You don't want a valve to open and feed a cold slug of
oil directly into your engine.
A while back, some one had developed a manual thermostat valve that could be adjusted from 0% to 100%, and various points in between. I havn't seen it for sale in a while.
On the street, you really don't need a cooler. Engine oil is heated by RPM's. In routine driving, it will be about the same temp as the coolant, give or take a little. If it's considerably hotter than coolant temp, there's a problem that needs to be solved with something besides a bigger cooler.
I completely divorced my oil cooling system from my oil pressure system. There's a couple of -10 ports on my oil pan, and an electric pump bolted to the frame rail. When I'm at the track, I run the electric pump to circulate oil through the cooler. No pressure loss through the engine block, either.
Occasionally I'll run it on the street. Last year we were driving from Wall, SD to Rapid City, SD. It was over 100*. So I ran the oil pump. Probably didn't need to, but it made me feel better.