Yes, all his albums have been remastered, a couple of times at least. Depends on what you're looking for. Oxygene was released in 2007 as a re-recording, but with the original instruments and it sounds practically identical to the 1976 release, but with awesome sound. You may want to get that one, with the 3D DVD.
Both Oxygene and Equinoxe got the Mobile Fidelity Sound Lab treatment in the 90s, on both vinyl and CD, and they sound truly excellent, with equally great packaging.
In France in the 90s, a series of remasters were released, identifiable by their golden border around the front artwork.
However, the remasters you are most likely to get your hands on is the Sony batch from 1997. Sony acquired the rights to all his back catalogue and remastered them all, from Oxygene through to Hong Kong (1994). These are also easily spotted if you see any Sony or Epic logos on them. However, since Jarre is not with Sony anymore, I doubt they do much in terms of making those CDs available, and many regular shops do not have them, so you're more likely to get them online.