smeg a42 ?

Without knowing the full circumstances, it's a little difficult to give better advice. We are looking at an Earth leakage, so proceed with caution.. Should your oven have been left unused for a long period the heating elements can absorb moisture and create a condition where the power can leak to the metal body of the oven. To fix this you can try disconnecting the earth connection from the power supply cable, insulate yourself from coming into contact with the oven by wearing rubber gloves, then power up the oven and allow to heat up for half an hour or so. Then switch off power at the wall and rewire the Earth connection.
This will dry out the elements and remove the moisture that was causing the RCD to trip. Please understand the potential dangers of attempting this fix, and treat the oven as being potentially live all the time the Earth is not connected.
If your oven hasn't been left unused for a long period, this suggestion is probably not relevant and you need to look for another wiring fault.

Rupert
December 2019