Yes fish oil supplements can be taken onto the airplane in your hand luggage or in your checked luggage.
You're allowed to bring supplements and other medicine in reasonable quantities onto the airplane either in your hand luggage or checked luggage.
You should bring enough supplements such as the fish oil in your hand luggage and if you need some more fish oil during your trip then you can put the extras in the checked luggage.
Or you could buy some more fish oil supplements when you get to your destination.
I've had no problem bringing my fish oil supplements onto the airplane in both my checked luggage or hand luggage.
I also bring other medications including cough medicine in sealed bottles when needed.