This study investigated the effect of self-assessment in Oral English Achievement and the moderating effect of preferred learning style. The study used the quasi-experiment research design. The population for this study consisted 24,769 senior secondary school students in class 2 (SS2) in Bayelsa state. The sample for this study consisted of 360 senior secondary school students in class 2 (SS2) of public secondary schools in Bayelsa State, Nigeria. Two instruments were used for the study. They were the ‘Oral English Achievement Test (OEAT)’ and the VARK Questionnaire. Face and content validity of the instrument was determined by giving it to three experts in Measurement and Evaluation specialty in the Department of Educational Psychology, Guidance and Counselling in University of Port Harcourt. Reliability of the instrument was determined using KR20. After analysis, a reliability coefficient of 0.79 was realized indicating that the instrument was highly reliable. On the other hand, the reliability of VARK was established using Cronbach Alpha method of reliability with an index of 0.80. Mean, standard deviation, and ANCOVA were used to analyze the data. Research question 1 and 2 were analyzed using mean and standard deviation while their corresponding hypotheses were analyzed using paired sample t-test and ANCOVA. Findings of the study revealed that there is a significant difference (p=0.00>0.05) in the mean (prettest and posttest) score of students in the self-assessment group based on their learning style in Bayelsa State, Nigeria. Based on these finding, it was recommended among others that students should be encouraged by the classroom teacher to carry out self-assessment intermittently since it has been seen that it improves the achievement of student especially in oral English.