OSGB feaures not aligning with OpenStreetMap basemap even though Data Frame and Features are all in OSGB?

Question

I have a task to report features in OSGB (British Nat Grid) over an OpenStreetMap basemap.

I have done this successfully in past with line up of features on basemap, but this time I cannot get the features correctly aligned.

Comparing the two (old test and new task) map files, I cannot see any difference in settings but one works and one does not. All features (vectors) are in OSGB and the dataframe for both is in OSGB, but something else must be causing the offset.

Anything subtle I might have missed in this new task.

I did spend a lot of time in the previous test exploring this but can't remember anything that seems missing now.

I have tried for the new set, creating new maps and adding alternately the base map and features first and in both cases making sure data frame is OSGB, but no difference to the result.

Answer 1

I just spotted one difference, which seems to have solved the problem.

Within the Data Frame --> Coordinate System tab, the Transformations had not been set for WGS84 -> OSGB.