Oil coming from the air box is usually a sign of overfilling (there's a line between the case and the air box for venting, excess oil can be pushed up this line).
Also, depending on the air filter you have installed, it could be a reusable one that gets cleaned and oiled instead of changed. I think the OEM filter is an oiled-foam reusable. Often times people over oil them, which can lead to excess oil in the air box.
Definitely check your oil level.