Split features maintaining ordnance survey map details in QGIS

I’m trying to split a polygon drawn over an ordnance survey map area that represents the total land of a farm, in order to create smaller polygons that represent individual fields. I wish to create the smaller polygons based on the exact field boundaries shown on the ordnance survey map.

When I select the polygon to split, it turns solid yellow, and I can no longer see the features on the map underneath. Is there are way to adjust the opacity of selected polygon in order to be able to cut along the ordnance survey map lines?

I’m aware the vector layer opacity can be altered via right click > properties > symbology. But when highlighting the polygon with the Select Features tool, to be cut using Split Features, it turns solid yellow.