THese may be planaria or tubifaria worms and they are not harmful to your fish but do indicate an overabundance of food. These guys only appear when there is a food source and the more of them there are the more food they have. Therefore we are probably looking at an overfeeding siuation which is common among fish like oscars. They are very messy eaters and tend to leave a lot of scrap arounf the tank. The fish may eat the worms themselves but if you step up the cleaning they will go away. HTH
get a clown pleco. The oscar might talorate one. If my LPS can have some baracuda live hapily with 4 plecos in a 30 gallon tank then heck why not try it