The Al Hajar stretch for about 700 km across the north of the country and rise to over 3,000m (Jebel Shams – mountain of sun) from the coastal plain. Sediments at the core were mainly laid down during the Late Permian to Late Cretaceous in the Tethys ocean basin that had resulted from the break-up of Gondwana.
The Arabian Plate collided with and pushed against the Iranian Plate, resulting in mountains chiefly made of Cretaceous limestones and ophiolites.
Rock outcrops in the Al Hajar Mountains, the Huqf and Dhofar span about 825 million years and includes at least three periods when the country was covered by ice.
Oman, located at the south-east corner of the Arabian plate, is being pushed slowly northward, as the Red Sea grows wider. The Al Hajar Mountains and valleys of Musandam are dramatic reminders of this: Oman is fairly quiescent tectonically but the Musandan experiences occasional tremors as the Arabian Plate collides with the Eurasian Plate (I remember coming back from holiday & finding bathroom tiles all over the floor from one of these tremors).
During the Cretaceous Period Oman was located adjacent to a subduction zone and a portion of the upper mantle along with overlying seafloor volcanic rocks were thrust over the continental crust. This obducted sequence of ultramafic to mafic rocks is the Semail Ophiolite complex. The ophiolite is locally rich in copper and chromite ore.
The interior plains of Oman are of young sedimentary rocks, wadi gravels, dune sands and salt flats. Beneath them is a several kilometre thick stack of older sedimentary rocks that host the country’s hydrocarbon resources.
Links for most of this come from – Encyclopædia Britannica & Wikipedia.