Evening primrose oil can cause some mild side effects including gastrointestinal upset and headache.
Cod liver oil has less severe complications such as nausea, diarrhea, flatulence and bloating.

There is very early evidence from animal studies that one of the main ingredients of evening primrose oil, gamma linolenic acid (omega-6 fatty acid), can be helpful for the treatment of GERD symptoms or prevention of its complications.

For which condition you want to take these supplements?

i wanted to try something for my acid reflux but i dont want the hernia to flare up .

Consumption of evening primrose oil won't cause hernia to flare up.
You can have hiatus hernia, acquired or congenital (born like this).
Conditions that can make your hernia flare up are those related to increased abdominal pressure, like pregnancy, obesity, straining due to constipation.
Muscle weakening and loss of elasticity as people age is thought to predispose to hiatus hernia.
Patients can have reflux without a demonstrable hiatal hernia. When a hernia is present in a patient with symptomatic GERD, the hernia may worsen symptoms.

Best wishes!
Marija

can evening primrose etc make acid flare up?

As I already wrote, this oil can cause some mild stomach and bowel upset, but, also, contains acid that can be helpful for the treatment of GERD symptoms or prevention of its complications.