Was interesting trying ot identify this O class submarine. Photo taken from Tender HMS Forth in 1942 at Holy Loch Scotland.
Based on date of photograph i(1942) by then half the O class had been lost by august 1942, A few operated their entire war careers in the Mediterranean Several in the RAN. (Pacific and Indian Ocean). Remaining then by process of elimination, this submarine then likely is either HMS Oberon (N 21) or HMS Otus (N 92). Both these old boats were scrapped in 1946.
See the dark gray blue casing fairwater color.