Best treatment is frequent water changes and fin's will heal nicely without med's, assuming you either increase territorial landscape,provide more places for fish to retire to,remove fish or fishes that clearly won't co-exist due to differing water chemistry need's, aggression level's, and perform perhap's ,twice weekly 50% water changes till condition improves.
Small African's can and do get along for some time, but once they reach sexual maturity (4 month's or approx 4 inches),then thing's get rougher for subdominant fish.(Many don't stay small)
Subdominant fish (cichlid's),, when kept in number's,, often sport varying degree's of fin injury due to the pecking order which is ALWAY'S changing in African tank's where Males challenge males for said dominance, or right to breed with particular female's of like specie's, or closely related, which present's all new problem's with unpredictable beahivor,development in cross bred species.
In regard's to the Oscar
,, to get most out of this species, they would welcome soft to moderately hard <12 DGH pH around 7.4 which would not suit the African's for long as biological process in the tank over month's will favor the Oscar
more,,,and the African's less.
Is usually the case, when water chemisrtry does not fall in optimum range for both species or several species being kept.
One or the other,or a few more than other's, are much more prone to stress related weakened immune system which in turn,,,makes the fishes much more suceptible to parasitic,bacterial,pathogen's.
Is primary reason why med's don't often work, or when they app0ear to work,,,symptom's often return over and over, and folk's get frustrated.
Much oF the time,,problem's are self inflicted. People aren't familiar with fishes need's,can't readily identify sick fishes,keep fishes outside their desired ranges for GH,pH,temp, and select poor tankamtes with little to no research before hand.
Not trying to be coarse,,just offering some truth's for consideration.