Not quite sure what is meant by “os map” (and pretty late to ask about the OP) but generally you can get arbitrary polygon extracts on https://extract.bbbike.org/ Once you moved the map to your area of interest and click on “here” the website starts with a rectangle. You can resize it, move it and add points to its contour. Available output formats include a variety from pure osm data to Garmin and Android compatible maps.