I have a briggs and stratton 22 inch cut 3.5 hp self propelled push mower my question is where do you put the oil in at ?

Here are some examples:

Google Image Result for http://www.jackssmallengines.com/Graphics/oil.jpg (http://images.google.com/imgres?imgurl=http://www.jackssmallengines.com/Graphics/oil.jpg&imgrefurl=http://www.jackssmallengines.com/Oil.cfm&h=191&w=357&sz=11&hl=en&start=15&tbnid=54rjqlLRn9nIKM:&tbnh=65&tbnw=121&prev=/images%3Fq%3Dlawn%2Bmower%2Boil%2Bfiller%2Bplug%26 gbv%3D2%26hl%3Den%26client%3Dfirefox-a%26rls%3Dorg.mozilla:en-US:official%26sa%3DG)

You typically use 30 W ND or non-detergent oil in the lawn mower or you can purchase 4 cycle oil.

There will either be a plug at the base of the engine or a large cap with a picture of an oil can. This has a dip stick in. Check the oil every day you use the mower.