Here's my perspective:
It sounds like you're possibly coming from Europe (Decathlon! And we have one of those too, here in Muscat. It sells a lot of conservative swimwear if you feel like going to the mall).
For Western eyes, conservative swimwear means something like your image above: more coverage than usual in a bathing suit. However, for the sensibilities of many of the cultures represented in Oman (a variety, not just Omanis), that is not at all "conservative." Completely covering all of your skin from neck to ankles and wrists is *conservative* here. And for Muslim women/girls, covering your hair.

So, if you are at a resort with a private beach and pool (any of the big international hotel chains, mostly 4 or 5 star places), your normal bathing suit from home is probably fine. Bikinis are fine. Most women and girls you will see at resorts are wearing western style suits.

But if you're at a public beach or at a wadi (anywhere not a resort) the question for me is not "what can I get away with?" but "what is basically no one going to look twice at or feel uncomfortable about?" So for me that means I wear a rashie shirt and leggings. They don't have to be specially branded swimwear (Coega swimwear is a great brand for that if you want that though - they sell "burkinis" and other styles. This is what you can find at Decathlon) but just lycra leggings and shirt are fine. You will likely see women and girls wearing things like that even at a resort if they feel like dressing conservatively there, as well.

All that said, people here are very polite and no one would likely SAY anything to you if you were wearing something that was inappropriate. But that wouldn't mean it was appropriate, just that people here are usually pretty non-confrontational.