The description from the Armory describes the sights on the Officer's Model as follows-
"The Rifle has a plain "buckhorn" sight on the barrel,graduated like the service rifle sight,and also has peep and globe sights."
The rear sight would have typically followed the pattern of Model 1873,1877,and 1879 rear sights used on rifles.Whichever one was in use at the time the OM was manufactured.Although it is possible a Buffington sight might be found on an Officer's Model it is unlikely as the last production of the OM was in 1885 and the Buffington sight was not approved for use until August,1886.The graduations on a standard rifle Buffington sight would be off a good bit if mounted on your OM due to the differences in the barrel length and height of existing front sight.