I'm trying to find an Op-rod for an M1 Garand and have several offers.  All have minor piting along the shaft of the rod.  I'm just wondering, what is considered "Out of Limits" when it comes to op-rod pitting?

Pitting was probably from use of corrosive ammo sometime in its life. As long as the pitting doesn't make the metal weaker, cracks or holes, it is mostly cosmetic. If sandblasted and refinished, may be less likely to be noticed.

I would be more concerned with the piston tip and op-rod tab being within spec. If the piston is undersized (less than .525 inches) you may have gas cycling issues. The tab (near the handle) not being within spec could cause the op-rod to dismount from the rifle while firing.

The op-rod should pass the "tilt test" in your rifle so parts do not bind.

If the op-rod looks beat down, it may be better to purchase one in better condition.

Dupage Trading has good inspec Op Rods for $80.00
If you have a Op Rod now but is out of spec you can have a new piston installed, tab rebuilt and bent back to proper specs by Columbus Machine Works Inc
I have had them rebuild three for me and was pleased with their work