I hate to ask but I can't seem to find this fitting.
Valve covers have a 1-1/2" female hole and I need to find an adapter to go to -12 AN male for my vacuum pump.
So I need a 1-1/2" O ring male x -12 AN male

Might be a #20 fitting. #16 is more common for a valve cover but what you are describing is larger than #16.

I just looked in the Aeroquip catalogue. Unless i am overlooking something (quite easily) I am not finding a 1-1./2 SAE O-ring fitting.1-5/16, 1-9/16, 1-5/8, but no 1-1/2. Maybe double check the size in the valve cover. For the -12 AN hose, do you want 37 degree flare, 45 degree??? I don't think SAE 1-1/2 exists.

-20AN is a tad too big.

I have a Fragola -16AN to -12AN male and the -16AN side falls right in.

I put my caliper on the oil fill threaded cap and it is definitely 1-1/2".
My only other option is to drill out the top of the oil fill cap and tap a 1/2" NPT or a 3/4" npt hole and get a adapter fitting and thread it in the cap.

Pictures?

1 1/2 thread is used on plumbing. Seems an odd choice for a valve cover. Where did you get the valve covers? Are the billet fab or something?

Check out this link: https://www.hydraulicsdirect.com/Fitting-Thread-Chart-s/1934.htm#SAE-STO-R
If you are measuring to the inside of the threads you could easily have a 1-5/8 SAE O-ring thread in the valve cover. Alternative is it is not a SAE o-ring fitting in the valve cover.
Unfortunately the link does not give the inside thread thread diameter for female threads.
Your idea of just tapping the oil filler cap might be the simplest.
Good luck, agreed on the pictures request.