Oil in the air box

Did you put the right amount in, I believe that if you over fill it, it will come out in the airbox

Sounds to me like you have a TRX350 Rancher .. ?

what does the rancher part have to di with it?

crankcase vent will spurt oil into the airbox if the motor is overfilled.

Had a friend with the same prob. The dealer said to run the bike then drain and fill the oil. I said don't do that because the oil cooler will fill up and you won't get all the oil out just draining it , so when you fill it with the right amount it's overfull because of the xtra oil still in the cooler. I said get the bike hot and let it sit overnight then drain and fill with the right amount ( Service manual specs.After draining/filter change 2.6USqt)
He has no more problems at all now.

I just got a 500 Fourman 4x4 a couple months ago.When I was overlooking it the first day I noticed in my airbox drain a oily fluid inside of it.After doing some observing I came to the result that the factory when prepping the atv sprayed or poured way to much air filter oil for the foam cover and it was in the drain,all on the sides of the airbox and actually I found some residue inside the carb.You didn't state if your bike was new or an older atv,so if you just bought it new it could be the same situation.Takes a couple wrags and cleaner to remove,good luck!!!