There are a few reasons why oil would get into your air box. It's fairly common, not to worry. But here are some reasons why and fixes:
Make sure your crank case vent hose is not pinched and that it's routed correctly. It should feed into air box.
If crankcase oil level is over filled, excess oil will be pumped out of crankcase vent hose and into air box. Engines do not like to be overfilled with oil, so make certain level is correct.
Some bikes, when run at high rpm for a long period, (motorway) can pump oil into air box. If your rings are worn, then this can be more common. (tech term: Blow by)
If the air filter is OVER-oiled, then excess will run down into air box. A sticky foam filter oil is best ... use sparingly.
Keep air filter and Air Box Clean.