With the low suggested torque value of the oil pump cover bolts the thicker "wrong" o-ring won't compress enough leaving you with too much clearance in the oil pump and low HOT oil pressure. Not optimum, but over the short time that you ran it, there was probably little or no damage. You caught the problem early, good for you. BUT if anything was damaged, it would have been a rod bearing, which is easy to check and inexpensive to replace. As a quick check, remove the oil filter, cut it apart, unroll it and read it-- shiny and/or coppery flakes are un-good.
'73 R60/5 Toaster
Luddite. Not just a philosophy, a way of life...