Hi Brigitte,
In all Arab countries, providing copies of your documents is something completely normal. The same goes for photos. When I first moved to UAE for example, I had to provide 6 copies of my passport as well as a total of 18 photos to get all papers issued from visa to residency to medical insurance. It is such a normal thing to do that people stopped questioning the whole idea of it. Even when I applied for a PO box I had to deliver 2 copies of my passport as well as 2 photos. In most western countries, this is exactly where data protection laws or guidelines would kick in, yet don't expect anything like this in a Gulf country.
Now, when renting a car in Oman they will categorically scan or copy your passport and driving license as part of their normal process. The only thing I am wondering about is why they want to receive it prior to your rental. Are you renting with a big company or 'just' a small local one?
Send me a private message if you like.