When the cats get "big" and you have mentioned that, they cannot groom the back of their body and tail area.

Sheba at one time was like this. When she was 16 I took her to the vet with the dandruff like you describe.

After testing for diabetes, etc, she was cleared of being ill. The vet gave us some prednisone for the skin, which I used for a short while, and gave a shrimp (human frozen tail on) as a treat for after pilling her. The pred was a small dose compared to what Oreo is getting now.

Slowly, Sheba started to beg for the shrimp when the 5 day pill time ended. So I continued it... being the soft touch I am.

That habit grew to 3 shrimps once a day as a treat, and Sheba lost weight, her fur perked up, and all the dry skin went away. This took about 2 yrs...slowly. The vet was AMAZED when I brought her back! Totally AMAZED. I called it the kittie Atkins diet... high protein, satisfied her and she ate less kibble etc.

We normally use Eukanuba which had Omega-3's added...to the kibble anyway. Sheba continued and lived far longer than predicted! By FAR!. She died just about at 24, (a couple of weeks short).

You can mix the salmon oil in food... but our cats won't touch it that way. Oils get rancid-- "off" very quickly. I'd store in the frig, but it might be "off" already.

I am now putting one large shrimp in Oreo's special tuna in oil, to help her with the increased appetite the prednisolone is causing. I figured if it could help Sheba, it may help Oreo, too.

Some fish oils have an off --scent. And our cats will not take them mixed in food. I've tried.

I'd mix 1/2 of what you intend at first for the real dose, and mix well into some soft food that has a fish base. If she turns her nose up, then you may have to do human tuna in oil to get her going.

Oreo won't eat anything with an off scent. She is hugely piccy.
So she refused the shrimp alone--as it was "new" (and maybe reminded her of Sheba--Who knows!). I chopped it finely up and mixed in her tuna, and it went down the hatch in a blink of an eye!

Now, your kitty is already "chunky" so the oil form of tuna is maybe not needed. You could try the tuna in water, not albacore, but the cheap light tuna for humans. Albacore is higher in mercury contaminants.

Look at the kibble you give, and see if it has the Omega-3's in it. Most premium foods do. IAMs was one of the first cat food places to put them in. (I found them a decade ago).

So bottom line. If the oil has been on a shelf for a long time, the cat may refuse it. Sniff it yourself and see if it seems chemically off. Keep in frig once opened. Check the exp date.
Foods for animals normally are not supervised as closely as people things in general. So it is possible it might be "off" already.