Twee - If they're solid doors you should be okay, check how much thickness is on the door sides by looking at the top or bottom of the door to see how far in it's joint forms with the cross rails..

http://www.directdoors.com/info/diy/fitting-a-door

From above link - You can in most cases cut solid timber Exterior doors to fit a smaller size opening by up to 20mm (3/4") from each of the four edges. Please allow the new door to sit somewhere dry overnight to acclimatise. Do not stand the door against an external wall as this can encourage it to twist or bow. Try and decorate the door before fitting and choose a dry day to fit it.

Anyone advise me on how much I can take off the side of some replacement Oak doors ?

I have measured up and found that some of the new doors need to be shaved by as much as 20mm per side - is this too much for a door ?

They are solid carcases not "cardboard internals"