* = Apparently, a cloth rag draped over the side of the conning tower cut off the top of the large "O" painted on the side, giving the appearance of a "U" to the periscope of HM s/m Triton.

1:600 HM submarine Oberon. Note the original bull-nosed bow. © Andrew Arthur

1:600 picture of the latter groups. The conning tower was modified, and the bow was longer and had a ram profile. © Andrew Arthur

These boats were built for operations in the Far East, because of the lapse of an Anglo-Japanese treaty in 1922, it was realised that dditional, modern units would have to be built for service here. They all served in this area until the outbreak of the war.
Compared to the "L" class, they were 40 feet longer, 4 feet more beamier, 2 knots slower, had twice the torpedo armament and a much greater radius of action. The torpedo factor was what stood them apart from the "L" class, the latter were quite capable of reaching and patrolling the Far East, but could not remain fro long, because of limited amounts of reloads for their torpedoes : the "O" class could fire each tube twice.
Each shaft had two electric motors powering it, arranged in tandem, and could be used singly for extended running or all four at once for high speed burst. This arrangement remained standard in the RN ever after. They were also the first RN subs to be fitted with Asdic as standard, a Type 116 set being carried.
An early problem of  the class was fuel seepage from the external tanks which were of riveted construction.
Originally called "O.1", Oberon was very attractive to the RAN, and they ordered two, AO.1 & AO2, to a slightly different design. the main feature was the ram-bow, Oberon having a bull nosed stem. Three similar units were built by Vickers for Chile.
As an economy measure, AO.1 & AO.2 were returned to the RN in 1931, and renamed Oxley and Otway respectively.
The second, Odin, batch had the ram-bow of the "AO"'s, but a slightly higher surfaced speed. They also had their hydroplanes moved to the top of the hull. Although this meant a slightly slower diving time, they were easier to maintence, were less prone to damage, and could be retraced to lie flush with the hull when not in use, allowing the sub to get very close to a pier.
At the outbreak of war they were dispatched to serve in the Mediterranean for which they were wholly unsuited for, and they were soon returned hime and used in training duties.