IMO this is why oscar's should'nt be a beginner's choice of cichlid , they need extremelly dedicated care and large large tank's 125gal at a minimum for over all water health and clarity..
HITH (hole in the head)
WATER!!!.. must maintain your water,vacum often(top off) drop out my be 2''of water after feeding's as a quick clean up it will help along with two water change's a week 25 and 25 to make it a total of 50 gallon's you still have good bacteria to think about too put them on day's like monday and friday's , you will cycle more water this way and stress the fish less by not doing a massive change at one time that really is in emergency stiuation's .. stress coat to help the fish's slime coat aswell .. and a good amonnia remover ( amonnia rock's not chip's and resin pad's)
TEMPATURE... temp of the tank must remain consistant. 74 to 78 degree's any were in this range is fine but it must be 75 degree's or 77 degree's not raise and drop often this will make him more likely to show signs..
POOR DIET!!!! YOU HAVE TO MIX IT UP BROTHA...

EX: good type of pellet food , live type food (NO STORE BOUGHT FEEDER'S ) raise or catch them yourself , INSECT'S try to gut load them with vegatable's first..or local cought verse's bought is better still (cricket's moth's beetle's centapeade's) try catching smallfish in the local river's here in C.T net along theedge's for small baby's depending on the size of your fish.. table scrap's yes table scrap's small ( depend's on size of your fish) chunk's of cooked fish shrimp raw?frozen/cooked , steak , pork , chicken all excepted by my old oscar and little was wasted ..
CARBON!!! is a major contributor to fish develpoing HITH, if you can swap out the carbon for mabe and alternatvie type carbon but not actual carbon or eliminate only carbon as a chemical filter.. and go heavy on biological and mechanical with a wet/dry sump system or a hang on back that allow's you to use media and customize it for you oscar..
HITH.. is a faster killer of jack dempsey's , jaguar's then oscar's .. they (oscar's) can live with far worse case's, but unless treated they will die..
HITH is not cureable it will allway's be in the tank , but you can heal the sore's and keep it at idle..