Oil in air box ninja 250?

There's a hose that runs from the top of the engine/transmission to the airbox. This is the Crankcase vent hose. Pressure in the crankcase caused by blowby is relieved into the airbox to reduce pollution. It's 100% normal for there to by oil in the bottom of the airbox Running the engine for long periods of time above 8,000RPM will increase the amount of oil in the airbox. There's a little drain hose on the bottom of the airbox, just pull the plug and let the oil drain out into the pan everytime you do an oil change.