OS maps have a license requirement, and hence are not in OSMand by default, but here is how to add them (by getting a free license).
Create a new map layer, by first choosing "Install more" and selecting a free included microsoft maps layer. Next "define/edit" one of them. Choose one of the microsoft ones from the dropdown, and give it a new name.
Change the URL to be this: (yes, I know it's code, but in this case you can put code in the tile URL box).
public static String eqt(int z, int x, int y) {static final char[] NUM_CHAR = { '0', '1', '2', '3' };char[] tn = new char[z];for (int i = z - 1; i >= 0; i--) {int num = (x % 2) | ((y % 2) << 1);tn[i] = NUM_CHAR[num];x >>= 1;y >>= 1;}return new String(tn);} String getTileUrl(int z, int x, int y) { return "http://ak.t2.tiles.virtualearth.net/tiles/r" + eqt(z,x,y) + ".png" + "?g=3675&productSet=mmOS&key=YOUR_KEY_HERE";}
Replace YOUR_KEY_HERE with your license key from the first step.
Enjoy nice ordinance survey maps! (This should also work for any other custom map types that require licenses).