Background/Objectives: Currently, Oman lacks a valid culture-specific food frequency questionnaire (FFQ) for adults. This research aimed to develop and validate a reliable (FFQ) specifically for Omani adults. The study was conducted in two phases, employing both qualitative and quantitative methods. Phase 1 was the development of the (FFQ); in this phase the Diet History Questionnaire II (DHQ II) was adapted to develop the Omani Food Frequency Questionnaire (OFFQ), translated to the Arabic language, back-translated to English, pilot tested, and then refined to be used in the reliability study. Phase 2 was the reliability study, in this phase, the OFFQ was administered twice in Arabic with the second being filled one to two weeks after the first questionnaire. Methods: A convenience sample of healthy Omani adults (n = 62) was recruited from Sultan Qaboos University (SQU) employees and students. To assess the reliability of the OFFQ both internal consistency and test–retest reliability were assessed. Internal consistency was assessed by conducting Cronbach’s α test, while test–retest reliability was assessed by comparing the median intake of food groups by calculating weighted kappa (K_W) and intraclass correlation coefficients (ICCs). Results: Weighted kappa (K_W) between the two OFFQ administrations showed fair to moderate agreement with (K_W) values ranging from 0.38 to 0.60 for questions assessing the frequency of intake. While the median (K_W) values between the two OFFQ administrations ranged from 0.26 to 0.58 for questions assessing portion size. The majority of food groups showed moderate to good test–retest reliability with median (ICCs) for food groups ranging from 0.57 to 0.80 for frequency questions and from 0.40 to 0.82 for portion questions. Conclusions: In conclusion, the newly developed OFFQ was deemed valid for the Omani dietary culture and is a reliable tool that can measure habitual dietary intake among Omani adults as evidenced by the reliability coefficients ranging from moderate to good agreement for the most food items and food groups. Future studies are warranted to assess the relative validity of the OFFQ and the inclusion of diverse demographic groups and a larger sample size.