This study investigates the relationship between the use of OK (or okay) and language choices by two native-speaking (NS) and two non-native-speaking (NNS) high school teachers in Taiwan. Four sessions were video-recorded and transcribed to examine OK in the teachers’ utterances in English, Mandarin and when switching codes. The functional categories for 286 instances of OK in the 24,754-word total are described from the bottom up and assessed for reliability to reduce the subjectivity of category judgments. We found that NS teachers use OK most in their English utterances, whereas NNS teachers do so in their Mandarin utterances. All teachers tend to use it to mark discourse boundaries in various linguistic contexts. OK also has a limited role in code-switches. It mainly appears to mark a discourse boundary or to connect English, as the students’ second language, with translation into Mandarin, as their first language.