Been fitting doors for a long time now,and every customer says theyre just standard sizes,i
have yet to know what a standard size is,here are Metric doors,taller and fatter on thickness than an imperial door, 80 inches high,and imperial doors, thinner and usually 78 or 80 inches high, 24 27 30 32 or 33 wide, then there are a few homes with 29 inch door openings.
Most of the Oak doors are a chipboard core,slightly lighter than the MDF core doors, both have their problems with veneer lift,veneer spliting at edges, thin side lippings,intolerance to some varnishes/stains oils,add on the weight, are your frames solid and not split,will they take weight of the doors,and finally the MDF core doors, once youve put a screw into the side, the very long screws required will sometimes pull out, re splinting them doesnt always work just splits the door open even further, think putting your finger in between the pages of a book , it can easily be pulled out and makes the book thicker.
When you go to buy a door or doors, easure the opening size of frame, and remember the lipping thickness, plane to much off and your through to the pine frame or mdf, looks terrible, take door out of rack carefully theyre heavy, tilt on side and look at short edge top or bottom, and you can see the lipping and the softwood frame usually, may need to remove the plastic bumper first.
Finally ask your fitter if they have fitted oak faced doors before, and how many they have done and any problems they have had.