I don't know what you feed yours now, but I have a long post saved on good oscar foods. So i'll post that up for you.
There is a lot of other stuff out there to feed them though. I'll split the food into 5 categories: Pellets, Live, freeze-dried, and frozen, and other
Pellets: These should be the staple of the oscar's diet. People recommend a number of things. I don't know what you have available in Malaysia. You mentioned hikari. Hikari cichlid gold works fine. Latest craze is New Life Spectrum foods. It is all natural and it is made to bring out awesome colors in fish. People say the difference between their fish's colors using NLS after hikari is like night and day. Anyway, if you can get your hands on that, try it out. If not, Hikari works fine.
Live: If you can't get your oscar to eat anything else, this is what will do it
1) Crickets-You can buy crickets for very cheap from your local petco or other stores. Most oscars will take them very willingly. For an added bonus, buy an extra 10g tank. Keep the crickets in there for a few days with cricket food in there. A number of things will work. Obviously, it puts more nutrients into the cricket. Ask if your interested about cricket food.
2) Worms-cichlids will slurp up worms like anything. They wiggle and they sink. Drop these in front of your 13" oscar's face, I can almost guarantee it'll inhale it. There are a few kinds of worms.
a) Mealworms: All of my fish eat them. Oscar, dempsey, silver dollars. They can be considered a bit fatty, so they shouldn't really be a staple diet. But if all else fails with the 13-incher, its better than feeder fish. They will die VERY quickly in the water. 10-20 seconds maybe. So be sure that yours oscars are watching.
b) Waxworms-Very similar to mealworms. Except they are very soft and fun to play with. I've never personally used them, so I'm not 100% sure if they sink or float and how long they'll live in water.
c) Earthworms-Thing about what every fishing store sells. Fish like earthworms. They are very nutricious. They'll live underwater for up to 2 hours! The only downside to them is that that dirt in their bodies will dirty the water. You can get your hands dirty and squeeze that stuff out if you wish. If you are interested, ask about the technique. Earthworms are very breedable. Get another 10g tank, fill it with 6" of dirt, add 100 worms, little food, keep it damp, and you'll have yourself a quickly replenishing supply of worms.
http://www.oldgrowth.org/compost/wormfaq
d) There are other types of worms that I have not accounted for. Thats cause I don't know enough about them.
3) Crayfish-These might be known to your as crawdads or a number of other things. In case you don't know, they are crustaceons. They're basically freshwater lobsters. Oscars LOVE them. If you don't take my word for it, go to
www.aquahobby.com then visit the gallery and go to the oscar link. There are a bunch of crayfish feeding stories in there.
Freeze-Dried -They are healthy. It makes up the staple diet of my fish. I'll list a few...not much to describe though
1) Brine shrimp
2) Blood worms-I find these kinda worthless. Oscar doesnt' pay much attention to them as they are very very small.
3) Krill-All of my fish love the krill. If i drop pellets, brine shrimp, and krill in the water, guess which goes first.
There are other freeze-dried foods out there. There are probably sitting downstairs by my tank which I don't remember.
Frozen-I like frozen. My fish seem to attack it more vigorously than the freeze-dried. Probably cause they dont' get it as often.
1) Brine shrimp-The problem with this is that the brine shrimp are TINY. At the size your oscars are at, i'm not so sure they'd go after the little shrimps. If if you did try this, you'd have to make sure that the cube is still kinda frozen, so an oscar can take it all in 1 bite.
2) Krill-Good stuff. Although its harder to chew for them than the freeze dried (i think). They're not as anxious to eat after eating frozen krill than after freeze dried.
3) Silversides-Little minnow like things. My oscar is the only fish in the tank that will eat them. I'm not sure how much nutrician is in it. So you might want to soak the silversides in a vitamin supplement before feeding them. Ask for more details if you want.
4) Beefheart-Took my oscar a while to get into it, but now it loves beefheart. It might be a bit fatty, so not really advisable as a staple diet.
Other -Other is other, don't know how else to say it.
1) Cocktail shrimp-so long as the shrimp aren't flavored with anything, you're fish will LOVE it. I chopped it up for mine. My oscar is only 5" long though.
2) Fish fillets-i've heard of plenty of carniverous fish eating this stuff. I'm sure oscars are no exception. Is if you're ever gonna have fish for dinner, maybe cut of a chunk before you season it. I havent' done it yet.
3) Cucumber/other veggies-When I would drop cucumber slices in for my pleco to suck on, they would float. When the lights went out, my oscar would go vertical at the cucumber slices and bite off pieces of it. A few people on this board feed their oscars veggies. So if you can get yours to eat the greens, i'd say go for it.