The current casual staple for any Omani girl's wardrobe for this year last was the "peasant" abaya. Lace smocking and panels across the chest and down the arms, and drawstring rope belt at the waist. I don't like how some girl's don't bother to have it sewn so that it doesn't make the curves of the butt obvious but I own more than one abaya in this particular style, all black on black, and I sewed the drawstring so it doesn't pull. The one pictured is a brand name version, which means the lace, fabric, and detailing are of better quality materials and was 60 OMR. Others are 25 OMR.
The key to this style, beyond the drawstring belted waist and lace panel smocking is there are usuaully tiers or pleating on the bottom of the abaya which gives it a dress-feel.
I haven't really seen any of the better quality "peasant" abayas available online yet, just the cheap-o fabric ones I pay 9 OMR for over-priced to like 100 USD, which is a crime, seriously. So beyond what is costs, we wear these generally for work and casual stuff, nothing too fancy.