i am thinking of getting a 10 to 13inch pleco to go in my 70G tank with 5inch oscer will the pleco harm the oscer in any way with it being so much bigger or will they leave each other alone

Unfortunately your tank is not big enough for both an Oscar and a large pleco. Plecos are incredibly messy creatures that require large tanks. This is one fish that will send your water quality downhill very quickly. That's why you see so many of them in the fish stores. People buy them as small fish not realising just how big they get. Personally, I would strongly advise you not to get one of these fish.

ok thanks would u be able to advise me on just what type and size of fish to get thanks.

Firstly, I'm not an expert on plecs, the less I have to do with them the better. All I can tell you is the majority of species you find in aquatic stores for very little money are the ones that get very big. They're absolutely loads different species of plec that don't grow anywhere near as big as the common species. However, it's finding them that can be difficult. Also, some of them are incredibly expensive. If you are dead set on getting one, I know for certain that the gold nugget Plecos reaches around seven or 8 inches maximum. However, I think I paid about £29 for the one I had a while back. Unfortunately, it died. Why not visit Plecofanatics.com and get some advice. They are a very friendly bunch of individuals who will give you some advice. But remember, your tank is not big enough for one of the big ones whatever anyone else says.

yes thanks but could u tell me what fish apart from pleco would mix with my oscer thanks

i am not sure if your oscar had established territorial
ownership.... i can suggest angel fish.... i have one with
my two oscars and two pangasius.