Precision and detail. The most precise thing on an OS map is an intersection of two grid lines. Assuming these lines are printed 0.1mm thick, on a 1:25,000 map an intersection represents an area of 2.5 metres square. Nothing else can be located more precisely than that, even though the survey itself is more accurate most lines on the map are drawn much more thickly than 0.1mm. Coincidentally 2.5m is similar to the level of precision available on a consumer GPS.
A node on OSM is defined to 7 decimal places, that is to the nearest centimetre, more or less. A GPS map display (old Etrex, worst case scenario) can zoom in to about 25m across the screen, or 7 pixels per metre. At this zoom a 2x2px point on the map represents 0.3m or about 1 foot square. It is easy for example to view a tracklog at this zoom and tell which side of the road the rider was riding along.
Of course OSM is a conglomerate of surveys of highly variable quality and provenance, any particular road junction could be loose/precise to within 100m (freehand drawing) or 10cm (commercial survey) and there is no way to know which when just viewing the map - but the precision range 5m to 1m probably covers most of it. 5m is ball-park comparable with OS 1:25,000.
Detail - one problem with OSM is that in some places (notably some university towns where the mapping is over-researched) there is far too much detail. Bus stops, post boxes, bike racks, real-ale pubs, all lovingly pinpointed. Apart from road mapping, in France for example they are obsessed with property boundaries, in fact these were mapped long before the roads were, back when OSM was evolving. In open country - yes OS mapping wins. Anywhere in the world outside of the UK - open country or not - OS is a fail.