There are lots of different worms, all of them will be readily accepted by fish. I suppose it depends on how big your Oscar is. If they are fairly small Oscars, then you are probably better off feeding the smaller worms. Problem is, I don't know what you call down in South Africa. Up here, we have Dendrbena worms, red worms and of course the biggest of them all, lob worms. I'm sure there are plenty of other worms, these are the ones I know because they are used in fresh water fishing. A lot of worms, especially Dendrbena and reds can be found in compost heaps, these will be perfectly okay for your fish. These are probably the easiest worms to harvest because lob worms are normally the type of worm you find on your lawn in the evening on a damp night. You can go out with a torch and wet the ground, this is often a good way of catching them.
Here in Devon where I live, they have a worm Charming contest every year
www.bbc.co.uk/devon/content/articles/200...ing_2006_event.shtml