Sorry Oyelowo fans! Rebels Report on Tumblr pointed out a Star Wars Blog comment where it was asked if David Oyelowo was voicing the Inquisitor and it was shot down by Lucasfilm:
There has been a lot of speculation about who plays in the Inquisitor. A well educated guess is that Brent Spiner, who played Data in Star Trek: The Next Generation will be voicing him after it was revealed he has a role in the series. Other think Max Von Sydow might play the Inquisitor in Star Wars: Episode VII and Star Wars Rebels. Regarding Sydow, I listened to his voice and then listened to the Rebels trailer and they do sound sort of the same when Sydow’s accent is British, however, it is totally inconclusive because the voice is processed (as it is a hologram in the clip).
Hopefully sooner than later we find out who is voicing the Inquisitor.