This embedded single-case study investigates four secondary school L2 teachers' perceptions of metacognitive oracy instruction (MOI) over a two-year Design-Based Research (DBR) project and examines the effects of the project on their declared practices almost a year after its completion. Questionnaires, learner notes, and interviews to the double were used to collect data. Data analysis included descriptive statistics and content analysis. The results indicate that the DBR project had an overall positive effect on three of the four teachers' perceptions of MOI. The study also found a link between the teachers' perceptions of MOI and their self-reported implementation in practice. The findings highlight difficulties teachers face in developing metacognitive practices, even when they believe in their efficacy and intend to implement them. The study offers insights into language teachers' adoption of metacognitive practices through DBR and can serve to inform teacher educators and researchers.