The o-rings you are asking about are inside the oil pan.They seal off the oil pick up lines.The fix is very involved as you have to suspend the engine/transmission assy,lower the subframe and remove the oil pan.It is not something the average weekend do it yourselfer would want to tackle.Really the only way to check for this is to use a oil pressure gauge that threads into the intermediate section of the block where the oil pressure switch goes and read the pressure values when the engine is running.