When we talk about any musician's rarities the first thing we have to decide is what is a rare song. Could all this kind of stuff be considered as "rare"?: unreleased songs, songs only published as singles, b-sides, songs only found on compilation albums, edits, extended versiones, different mixes, records only avaliable in special country editions, all of them, none of them... the thing is hard to say.
For me, I will consider an Oldfield raritie any song or record that could be hard for the fan to find, and due to the nowadays behaviour of the companies records, that will be any song never included on an official international album release or related and descatalogued albums.
First, you could find here some releases that for different considerations are official but can be considered, someway, as add-ons to Mike Oldfield discography as well as a list of albums related to Mike but, for one reason or another, outside of the official discography. Then I will give you a complete listing of songs not found on the official albums and its avaliablity including those only found on compilation albums as well as remixes & extended versions, edits, live recordings and DJ's mixes.
As I did on the single section, any song that is not found on the official albums, including compilations, will be marked with a , and those being just DJ's mixes will be distinguished with grayed color.