Yes, 11th Nice Classification. 

No; marks can be registered in Oman for all goods and services under classes 1 to 45 of the Nice Classification without any exclusion.

As per the new regulation implemented by the Omani trademark office in association with the Madrid protocol; the list of goods or/ and services of all applications should be detailed and specific.  It is not possible to file the trademark application with an entire class heading or with all goods/services. 

No.

The required documents should be submitted within 3 months from the filing date of the application.

No. a Separate application should be filed for each class of goods/services.

Yes.

The term of protection of a trademark is ten years from the date of filing the application.

No.

As per the Trademark Law, if a trademark registration is not renewed by the owner, the same mark will be allotted to another party only after a gap of three years.

No documents are required for amending the trademark until its registration.

No. However, the mark will be vulnerable to cancellation at the Trademark Office, if it is not effectively used for three years from the date of registration and it is vulnerable to cancellation at the Commercial Court if it is not used within five years from the registration. 

Yes.

Sale or offer for sale, or should be dealt with in some manner in Oman.

Yes.

Recording a license agreement is not compulsory, but highly recommended, as otherwise, it will not have any legal effect vis-à-vis third parties.

No. Applicant’s name, address, etc., can be amended in case of pending applications by submitting the appropriate documents.

It is possible to record an assignment with or without goodwill. A nominal amount (of say US$ 1.00) can be mentioned as the consideration. There is no tax based on the value mentioned.

A trademark application can be opposed within 60 days from the date of its publication in the Official Gazette.

Oman accepts only trade dress and 3D marks.

Yes, you may trademark be a city/country name if it is used as a brand name for your products or services.
3-Can a slogan be registered? Yes, can be registered if it's original, imaginative, paradoxical, surprising, thought-provoking, and unexpected; and/or combined with a distinctive word and/or logo.
4-Is the use of the trademark on one item sufficient to maintain the protection for all the goods covered by the registration? No, the use of the trademark on one item is not sufficient to maintain the protection for all the goods covered by the registration.

It’s possible to register a trademark without using it, however, trademark registrations in most jurisdictions may be vulnerable to cancellation in the event of non-use, usually within a period of either three or five years. 

The trademark should be unique, not similar to any other name, and consistent with the brand. 

Marking the trademark/copyright of any of those symbols is not mandatory in the MENA region, however, it’s advisable to use the same to let the others that the work is protected. 
TM Symbol may be used when the trademark is still under examination process and still not formally registered, while ® symbol may be used when the trademark is officially registered and © symbol can be used for registered copyright.