It has an Oldham postcode and a Rochdale STD code.
Until 1865 IIRC, the border passed through the middle of the town, symbolised by the frieze on the front of the Town Hall, which sat on the boundary. This anomaly occured because, from antiquity until the 18th Century, the valley bottom wasn't really habitable, and the settlements were based around the Mankinholes area on the southern shelf, and Heptonstall on the northern one, with the packhorse routes traversing the moors. When the Rochdale Canal was built, it drained the valley and the toll roads came through, with Todmorden being built on the junction of the Halifax, Burnley and Rochdale roads.
It was realised this wasn't a tenable position, so the border was moved about five miles west.
I did ask how Calderdale Council, which is entirely in Yorkshire, could demand Council Tax off me if they thought I lived in Lancashire.