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:

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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.