?English Oak veneer previously oiled

Hi. I am seeking some advise on how to proceed with finishing an old cupboard. Would be around 50 years old and may be english oak. It appears to have a veneer finish in that 4 triangular pieces join on the door, although the back of the door does not have this pattern. It was oiled some time ago and absorbed plenty of the oil and went much darker in colour. My question is; can this be sanded back (since the 'veneer' may be quite thin)? Should I use only very fine paper? I hope to stain and estapol it but don't know how the oil will affect how I proceed. I would appreciate advice. Thanks