I can see the OS open basemaps when using ArcMap desktop on my machine but they do not appear as an option in ArcPro or AGOL. Do I need to configure these, and how do I do this please?
You should be able to search for "OS Open Basemaps" under "All Portal" in ArcGIS Pro.
You can right click on these and select "Add and Open" and they will be the basemap in a new map tab.
In order for the OS Open Basemaps to appear in the basemap gallery in ArcGIS Pro, you need to make them available as a basemap in ArcGIS Online. To do so, you will need to create a new group that will serve as your basemap repository.
Change the basemap group for your ArcGIS Online organization.
Next, you need to create a web map with the desired service as the basemap.
The web map should now appear when you browse for basemaps in your ArcGIS Online organization in the map viewer as well as in ArcGIS Pro, given you are signed in with a member of the same organization.
Hope this helps!
Retrieving data ...