Only the most veteran Imperial TIE pilots were allowed to participate in the Oppressor flight test
program as only those personnel were deemed to have the tactical know-how to properly employ the ship as well as the loyalty to be trusted with such an expensive flight article.
During the Galactic Civil War, the Oppressor served in the fleet as part of a "Limited Fielding test
Initiative" and was considered a minor trial program by the Imperial Navy. Following the Galactic Civil War and the gradual decline of Imperial funds, it is believed that these extremely powerful ships gradually faded out of existence.
Rebel pilots found that when they encountered some of these ships in their bomber configuration, they were slow and easy to destroy, but not as easy as the slower TIE/sa bombers.
However, when used by a skilled Pilot
and equipped for anti-starfighter duty, the TIE Oppressor was deadly and could easily take out far more maneuverable craft. Pilots would use its reputation for slower speed as an advantage to trick the enemy into thinking the vessel was an easy target, then used snap speed changes to turn allowing sudden and unpredictable attacks against another fighter.