This will be hard to pin down with out pictures.

If
Quote
The oven power rating is for 240V, 20 amps

is correct than there is no need for a neutral and 12/2 cable used to feed kitchen outlets will be fine.

If the ovens rating is more than 20 amps the wire will be too small.

If the oven is rated 240/120 20 amps it will need a neutral along with 2 'hots'

Using the grounding conductor for neutral is not an option.

If you need 240/120 you will need to run a new wire.

Bob

Bob Badger
Construction & Maintenance Electrician
Massachusetts