A few snails is not going to hurt anyone, as long as it doesn't get out of hand. If you got fish like Clown Loach, they will love having snails in the tank, mind you, they will probably finish them off before too long. Some snails eat plants so if you like plants, remove the snails as soon as you see them, or quarantine plants for a few days and give them a good wash.
Now the snails that you don't want in your tank are called Malaysian trumpet Snails. Because there are live bearers, you only need one of these and you'll end up with millions of them before you know where you are. Some people actually introduce them into the tank because they aerate the substrate which can be good for plants. They can also eat uneaten food and dead fish that may have been missed. The snails normally only come out at night you don't see them during the day. However, they reproduce at an alarming rate.
What you've got to think about is they will also produce waste, the same as fish. The biggest problem with these type of snails as there are not many fish that can eat them. I can say this with confidence because I was infested a few years ago, I actually bought myself a couple of large Clown Loach thinking that they would feast on the snails and read the tank of them. How wrong was I, introducing the clown Loach didn't affect the population of snails at all, in fact they just kept coming. In the end, I removed all the substrate and sand in. I still occasionally find a snail in the filter now, I just don't get them in the tank because I don't think they like the sand.
If you like snails, get yourself a nice big apple snail, they will live happily in your tank and the Oscars won't be able to eat them.
