Apparently a one-off label. The only record I have been able to trace on L. O. was a single by Folk / Rock band The Strawbs, with Maddy Prior. It was called 'The King', and it came out in 1979 - or possibly early 1980 - with a catalogue number of LO-1. The label was owned by Kevin Wyatt Lown, who was the Strawbs' manager at the time; according to the Strawbs website the initials stand for Leyton Orient. The site gives a history of the single, and can be found here. Thanks to Robert Bowes for the scan.
Copyright 2009 Robert Lyons.