Oracy skills are considered as a problematic area for Thai students. Despite years of learning, students could not improve the skills at a satisfactory level due to internal and external hindrance such as cognitive load while communicating and insufficient practice time in class to achieve their communication (Flavell, 1979). In addition, to provide more in-class oracy practice time, blended-learning method is considered to be an effective platform in which students can learn grammar and vocabulary at their own pace outside of class. Oracy Building Instruction via Blended-learning Environment (OBIBLE) was developed to improve metacognitive awareness and oracy skills by using blended-learning environment. The research was conducted to investigate the effectiveness of the OBIBLE instruction on students' oracy skills and metacognition, and perception towards blended-learning environment. The instruction model has integrated activities to promote metacognitive awareness and oracy skills in the environment of a blended approach. Twenty-nine high school students in a public school in Rayong were chosen as the sample group. The research employed a single group experimental design method to estimate the effectiveness of the instruction. The data was analysed quantitively and qualitatively using pre- and post-test scores, task performance scores, questionnaires, and stimulated recall interviews. The finding shows that there was a significant improvement of the participants' English metacognitive awareness and oracy skills after taking the learning through OBIBLE, and that the students had awareness in the aspects of background, engagement, outcome and convenience of the instruction. This research provides empirical evidence for the effectiveness of blended-learning environment and gives useful insights for future students, teachers, and institutions in developing metacognitive awareness and teaching oracy skills of English.