you need to know the rating of the oven.
6mm will give you about 45amps but that is without reductions (depending on how the cable is run(i.e rockwall insulation has a big factor).
10mm is about 60amps and will almost 99% be big enough.
But are you taking this from the CU or are you joining this cable into the cooker socket near your cooker?, if this is the case then you will need to use the same cable as a minimum because there is no point running 10mm into a 6mm cooker spur!
If its lower in size you should really check your oven rating because double ovens will draw usually 20-30 amps (4mm minimum) singles you can get away with 13amps (2.5 minimum) 9 times out of 10.
But electric hobs are another ball game they will be big power consumers and you must make sure that your cable can handle the power being drawn before making any connections (diversity).
And if its both hob and oven thats where your 10mm will come into play.
Do not listen to people saying that it must be 6mm or 10mm if you use diversity then you can put whatever cable you choose there (as a part p registered electrician).