George Thomson printed O My Love ... to marshall's tune Wishaw's Favourite. I have Thomson's book and the melody is a stripped down version of marshall's "Mrs Hamilton of Wishaw" a stratspey which can be ssen here Mrs Hamilton of Wishaw
The reason it is difficult to scan in the words as are normally sung is that Thomson, in the high part, has repeated two words i.e. " So deeep, so deep in love am I"