Article 121 establishes the mechanism for a 'Foreign Tax Credit' to avoid international double taxation. When Omani-taxable income has already been taxed in a treaty country, the taxpayer is entitled to deduct the foreign tax paid from their Omani tax liability. This deduction applies only if the foreign tax relates to income that forms part of the Omani taxable income for that year. This ensures that the same profit is not fully taxed twice—once by the source country and once by Oman—promoting cross-border investment by Omani entities.