What happens if I open a vape cartridge with oil inside?

Whatever you do, don’t do that! You will most likely ruin your cartridge and flood the tank with oil, clogging the cartridge. The oil inside your cartridge stays put once the mouthpiece is put on for the first time after filling. If you open the cartridge by taking the mouthpiece off, the cartridge will lose the pressure that was keeping the oil inside the tank. 

How about I “explain it like I’m 5”. 

When you put a straw in your glass of milk, and then you put your finger on the top of the straw and take it out of the glass, the milk will magically stay in the straw. When you take your finger off the top of the straw, all the milk falls out and goes everywhere. 

Now pretend the base of the cartridge is a straw, and the mouthpiece is a finger. When you take the mouthpiece off the base, all the oil falls out. Just at a much slower rate because oil is much thicker than milk.