UK area code
Kidlington area code
Kidlington is covered by 01865 and 01869. It sits across a boundary between dialling areas, so which one applies depends on which part of Kidlington you mean.
01865The Oxford dialling areaSee what 01865 covers →01869The Bicester dialling areaSee what 01869 covers →
If you have the number already, the code it starts with is the answer — that's more reliable than the town name for a place split like this. Look it up.