Having read your additional suggestion about using Quo, and at the risk of being awarded the 'Pedant' badge, I should point out that if you overlay the grid from a transverse Mercator projection (as used by OS) on to a spherical Mercator projection (as used by OSM), the resulting grid lines will be neither exactly parallel nor exactly straight. The effect is small if you are mapping a small area (e.g. a town), but less so if you are mapping a county or a National Park. I can e-mail you an The image of below is the OS grid printed over an OSM map of most of the UK if you like, but I don't have any means of publishing it British Isles. The green line is 53°N on the Interweb.WGS-84.![OS grid over OSM map][1] [1]: http://help.openstreetmap.org/upfiles/Grid_Lines_1.bmp

Having read your additional suggestion about using Quo, and at the risk of being awarded the 'Pedant' badge, I should point out that if you overlay the grid from a transverse Mercator projection (as used by OS) on to a spherical Mercator projection (as used by OSM), the resulting grid lines will be neither exactly parallel nor exactly straight. The effect is small if you are mapping a small area (e.g. a town), but less so if you are mapping a county or a National Park. I can e-mail you an image of the OS grid printed over an OSM map of the UK if you like, but I don't have any means of publishing it on the Interweb.