It's hard to believe that so many ask for help with a problem yet give so little information about the equipment. There's no way to give a correct answer about an Onan Generator without knowing the complete model, serial number and "spec".
I have an Onan Micro Quiet 4000 generator on my MH. The capacity of the oil sump is 1.6 quarts (or 1 quart 19 ounces).
Because air can get trapped in the crankcase, it can effect the apparent level of oil when checking it through the oil fill hole. That trapped air can be negative or positive making the oil level appear to be high or low at the oil fill.
The best way to check the oil level is to do it first thing in the morning before starting it when pressures have equalized.
The best way to make sure you've got the correct amount of oil in the crankcase is to drain it and refill it with 1 quart, 19 ounces of fresh oil.