When he used the Arc of Infinity to cross over from the anti-matter universe, Omega materialised his TARDIS in Amsterdam as a brick wall in a crypt. The inside of the TARDIS appeared to be filled with bright, white light. Beyond this, it was similar to the Doctor's TARDIS, with a console room, side room, and roundels. The side room was large enough to be used as a prison cell.
The actual console and time rotor were non-existent, however, with controls instead being on the walls and the interior was illuminated by green light. The Ergon used its matter converter to transport people outside the TARDIS to the inside.
Having shifted the Arc to Amsterdam, Omega used a fusion converter from Gallifrey installed in the TARDIS and operated by the Ergon to transfer himself into "life" based on the Fifth Doctor's biodata. The Doctor interfered and Omega's conversion was left unstable, the TARDIS was destroyed in the overload.
Omega vowed to build another one and return to Gallifrey, but the Doctor used the matter converter to send Omega back to the anti-matter universe. What happened to the remains of the TARDIS was never explained. (TV: Arc of Infinity)