Electricians...Got a question.

I have a conventional home oven I plan on using in the garage for a powder coat oven. My water heater is in the garage and has 220v service. The box is mounted outside the drywall in the garage and has flexible conduit from the box to the water heater. 30 amp dedicated breaker in the fuse panel.

Can I tap into the water heater supply wiring for the oven and install a plug coming from that box? OR does the oven need a dedicated service as well?

needs to be by itself

By itself or else you'll trip the breaker or possibly smoke the wiring if the oven and the water heater are on att the same time.

you need 8/3 wire and a 40 amp breaker for the oven.
another difference is you need a neutral conductor for the oven.
HWT does not have one.