Hello I was planning to resign on my current job ...

We would like to inform you that according to the Omani Labor Law, to be able to change the employer then then termination shall be legal. So if there is violation by you for the contract and you didn’t complete the contract, then the employer can refuse granting you the NOC beside report absconding against you and ban you. So to take the NOC you have to notice the below:
Article 37:
If the contract is for an unlimited period, any party may choose to terminate it by giving the other party a thirty day written notice of termination in case of employees who receive their salaries monthly, and fifteen days for other employees unless it is agreed in the contract for a longer period.
If the contract is terminated without such period being observed, the party who terminates the contract will be obliged to pay to the other party a compensation equal to the gross salary of the notice period or the remaining part thereof.
Article 43:
The contract of employment may expire in any of the following events:-
Upon the expiry of its period or by completion of the work agreed upon.
Upon the death of the employee.
If the employee becomes incapable of carrying out his work.
Upon resignation or abandonment of work in accordance with the provisions this law.
If the illness of the employee requires discontinuation of work for a continuous or interrupted period of not less than ten weeks within one year.
The employees' incapacity or illness shall be proved by a medical certificate and proof of age shall be by the same certificate if it cannot be proved by birth certificate or on official printout of the same and the medical certificate must be issued by the Medical Committee, which will be convened by a decision from the Minister of Health in co-ordination with the Minister for the purpose of implementing the law, and such decision must contain the organisation of work procedures, and the decision of the said Committee shall be final.
The contract may not be terminated by the employer unless the employee attains, the age of sixty at least.
The employer shall, in case the contract is terminated for any of the reasons referred to above, pay the end of service gratuity provided for in Article (39) to the employee or his successors if the employee is not subject to the rules of the Law of Social Insurance
Therefore, if the termination was legal and the employer was arbitrary in granting you the NOC then you can file a complaint in the labor office and they will give you the permit to change the employer.