Why is there oil in the air filter area of the lawn mower?

that can be due to a number of different things. if it is a push mower, did it get turned on its side? if you turn a mower over always point the exhaust down. if you point the carb side down it will fill the air cleaner and filter with oil and it won't run. if it has oil in it and it hasn't been turned over then chances are you have excessive blow by through the crankcase vent tube.