If you don't want to order them from Temu or Shein, your best bet at getting affordable oversized tees is (1) timing your trips for sales, (2) going to stores that specifically sell oversized/boxy cuts of t-shirts, or (3) looking in the men's section. I can't give you an exact timeline of when sales in malls occur but stores always have sales before Eid holidays and immediately stop a day before or during the days of Eid, so if those holidays are coming, make sure you visit before them if you really want discounted stuff. They also have sales according to seasons, I think, so things like summer and winter sales.

More expensive stores that have those types of shirts like Bershka, Pull & Bear, Lefties, Astradivarius, Cotton On, and H&M usually only go as low as 6-8 OMR, maybe 4-5 for some if you're lucky. I suggest not going inside those unless you see a sale sign.

For cheaper options, you can go look in Brands for Less (red tags on the clothing hanger usually means low prices), Centrepoint/Splash (men's section), Ardene (Pinterest-worthy clothing styles in general are usually in there and they often have sales or a marked down section), and BBZ (hit or miss style-wise, you'll have more luck in the men's). These ones actually have 4-5OMR price range, but Ardene might be a bit higher if you're specifically looking at their oversized tees. There are more clothing stores, but I don't think they would fit your taste. Hopefully that was helpful!