A wall oven is built into cabinetry and is almost always hardwired, so the power must be cut at the breaker before you touch it. The real challenge is that it is heavy, dense, and set into an opening at chest height, which makes the removal itself the risky part.
Pro tip: Build or use a platform at the exact height of the opening so the oven slides straight out onto support.
Watch out: Wall ovens are hardwired. Cut the breaker and verify with a tester before touching the wiring.