Someone recently suggested on here that this common visa no longer exists. I also heard the same thing from a second source. There is no reference to it on the new Oman police website and one of the websites which shows information from Timatic says it's only valid when entering Oman through MCT.
Unfortunately I wasn't able to ask anyone about this on my recent transit through MCT (which did not involve passing through immigration - I would have if I only had to pay 5 riyals for 5 hours but I wasn't going to pay 20).
I believe you probably know this but you can't just "fly through DXB", you need to pass through immigration there as well.
visa waiver does exist and IS showing on Oman police website in both Arabic and English. both Dubai entry and exit stamp are required , can use to arrival at MCT also at Hatta land crossing.There is also similar visa waiver from Qatar to Oman for qualifying nationalities.I was told by Omani immigration (police) in MCT that this only applies on direct flights