A recent project for this site has involved walking between each pair of stations in OSIs and generating Google maps so people can see what is involved. The full list of OSIs can now be found on it’s own page while the general explanation has been re-written from scratch and renamed Interchanging Trains. In the list each entry with a map shows the distance in miles between the stations and that is a link to the map page. The map page lists just the entries applicable to that map (usually two) along with a Google map. The recommended route is shown with blue dots while Google tries to be helpful by adding one or more alternatives as grey lines. There are no maps where the two stations are linked by one concourse or are so close together as to make a map pointless.
For the statisticians out there, most OSIs are half a mile or less so should be walkable in under 10 minutes. The longest one is between Paddington and Marylebone NR stations at 0.8miles. Others over half a mile include the Bromley stations, Emabankment to Waterloo East, Euston to Kings Cross or St Pancras, and Ickenham to West Ruislip. Although not mapped, the shortest one is undoubtedly Southwark to Waterloo East which is about 20 yards.