The band recorded the songs in 1993 with only eight copies made to hand out, including early versions of tracks such as Columbia, Rock 'n' Roll Star and Married With Children, which were later given an official release.
The recordings of the eight tracks are now to be issued on a limited edition cassette as the band's early recordings are remastered to mark two decades since debut Definitely Maybe.
Oasis came to an end in 2009 after guitarist and main songwriter Noel Gallagher quit the line-up, after years of friction with his brother Liam, the group's singer.
They are now releasing new versions of the first album and its two successors (What's The Story) Morning Glory? and Be Here Now, which will also feature unreleased and rare recordings.
The Definitely Maybe album, coming out on May 19, will be issued in various formats including a box set and a three-CD version, which also includes songs from the demo. In addition there are live acoustic versions of hits such as Live Forever.
Fans were told via Twitter to expect an announcement at 9am this morning, which led some to speculate that the group may reform for the anniversary, although the Gallagher brothers - who are still not on speaking terms - have consistently said there were no plans to get back together.
Liam said recently: "There is no band. So, no, I won't be getting involved, anyway. If there is a reunion, I won't be in it."