OOPS stands for On/Off Purging System I believe. Basically, its where you screw the air in, but it has a lever or button to slowly release air into the marker, rather than slam it all in at once and risk breaking o-rings. It also bleeds (purges) the air when you turn it off, making the air bottle easier to unscrew.
Bottom line I think is the little bit of tubing, called a macroline, that runs from the OOPS to the regulator.
Edit - Ugh, beat me Lemon.
If you are going to buy an Ion, id recommend the following upgrades as a MUST:
- OOPS - As stated above.
- Feedneck - This is where your hopper sits in. The stock one isn't all that, and getting a nice adjustable one will make a huge difference.
Some optional upgrades I'd recommend if you have the cash:
A nice barrel kit, I have the Freak barrel set on mine. Will give you a better barrel to paint match.
An aftermarket trigger, nothing wrong with the stock one but you should be able to shoot faster with a better one.