UK area code
Oundle area code
Oundle is covered by 01780 and 01832. It sits across a boundary between dialling areas, so which one applies depends on which part of Oundle you mean.
01780The Stamford dialling areaSee what 01780 covers →01832The Clopton dialling areaSee what 01832 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.