The oil is coming out because the block is getting presurized by the normal blow by gasses which are part of how an engine runs.
To extract these gasses Toyota (and others) use a Positive Crankcase Ventilation (PCV) system.
What motor do you use?
It could be a blocked PCV valve. For an N/A motor it is normally in a hose running from either the side of the block or the rocker/cam cover that ends in the inlet tract. The gases and some oil gets sucked into the motor to be burnt. Fresh air is then drawn in through the breather.
If there is a lot of oil, then it could also be a sign of a very worn motor. The rings have lost their seal on the bores and most of the power is literally blown past the rings and out the breather hose.
If there is no PCV system then run a hose from the breather to a catch can, which will stop the enginebay from oiling up.
after i wrote this i saw YRHKIRA replied first which is basiclly the same so ill just leave my explaination aswell lol... it is quite normal to see oil come out of the breather, the reason it does that is because of the PCV (positive crankcase ventilation). On the compression stroke, a small amount of gases in the combustion chamber escapes the piston and sits at the bottom of your engine, when you drive your car harder it builds up pressure and forces oil back up to the top which is why you see oil come out of your hose. On my 2T-G i only have a little filter sitting on my rocker cover and that leaks after driving for abit. Im saving up for an oil catch can that way the oil can go straight to the can and you can just unscrew the bottom and let the oil drain every so often, beats cleaning your rocker cover and getting oil all over your engine bay and eventually in where your spark plugs are. hope that helps