Single Entry Visa:

Single entry visas are issued at all ports of entry in Country. To get this type of visa all you have to do is submit your identification with the documents listed here and pay a small fee, and you will be issued a visa valid for one month.
you can obtain these visas at Omani embassies and consulates.

If you are caught outstaying your visa´s validity period you will be penalized with daily fines which are almost equivalent to the fee you paid for the visa.

Multiple Entry Visa:

You can obtain this visa at all ports of entry in Oman after you fill out the application and pay an application fee. These visas are valid for a period of one year, though you may only remain in Oman for three weeks at a time. Means you can enter Oman in a year any time you want but at least three weeks must pass between each of your visits. Multiple-entry visas cannot be extended.

Express visa:

Omani Directorate General of Passports and Residency issue these visas through Omani consulates and embassies abroad. Foreigners nationals those who are not eligible for single or multiple entry visas have to apply for express visas.

Express visas are valid for only two weeks, and may not be extended. You will have to pay the same fines as multiple and single entry visas if violate any of the stipulated validity periods.

Read More at http://www.visaworker.com/oman/