This is what bluebag was invented for. Find an IGA (not sure if colesworth stock them) and look in the laundry aisle for 'laundry blue' - usually a bag or a block. Might be in the cleaning aisle. You wash item, and add the required amount of blueing to the rinse water. Dry as normal. It's an optical illusion, but makes the yellow tinge disappear. Might be worth considering if napisan or lookalike doesn't work.
Imo this is the only way to get whiter than whites without nasty chemicals.

Have a look into laundry stripping, I wear a lot of white undershirts and have a lot of light-coloured bedding and once a month (or sometimes more) I laundry strip everything.

If you haven’t already, try doing the oxy soak using hot water. It makes a huge difference. Also, check that your oxy powder has not expired. There will be a best before date somewhere on the package. The active ingredient degrades over time.

Just napisan mate. Put the item in a bucket with water and napisan overnight. Give it a stir with a big stick occasionally. Wash as normal without detergent.

Another way is to give it a quick scrub with Sards wonder soap and cold water. Chuck it in your machine with the soap still on it.

I just buy ivory colour linen.

I used an Asko front loader with a 90C temp setting and pre-soaking. These days I buy coloured sheets as I felt the maintenance for white ones had me wearing through them pretty quickly.

Question, how often do you wash bed sheets?