1. A compliment, meaning you have large amounts of oxygen, therefore having more brainpower.
2. An insult, meaning you have lots of oxygen in your head, and very little brain.
The big can also be replaced with other words describing size, such as massive or huge.
Good luck figuring out which.
Def 1: Dude, he just aced the test. Must have big oxygen levels.
Def 2: Wow, you play Fortnite? You have MASSIVE oxygen levels.
A Playstation owned and used by a console peasant
Bob: I just played on my Peasantstation with 14 gigs of GDDR90000 RAM at 300K...TOTALLY BEATS PC