Online teaching transition during COVID-19 school lockdown elicited challenges for teachers and schools across the globe. The existing literature on the impact of COVID-19 in the education sector is predominantly descriptive and focused on the difficulties faced by teachers during the process of transferring into online teaching, mainly in the higher education sector. This study adopted a mixed-method design to examine online teaching self-efficacy (TSE) during COVID-19, its associated factors and moderators. A sample of 351 Chinese school teachers retrospectively reported their online TSE at the beginning and end of COVID-19 school lockdown, out of which six were followed up for an in-depth interview. TSE for online instruction did not significantly increase ([beta] = 0.014, p > 0.05) whereas that for technology application increased significantly ([beta] = 0.231, p < 0.01). Lack of experience in online teaching, separation of teachers from students, school administrative process and unsatisfactory student academic performance were identified as the major associated factors. A moderation effect of adaptability and teacher burnout on the change in online TSE were examined, of which passion burnout was the only significant moderator toward the change in online TSE. The study thus concluded that teachers' online TSE for technology application increased among Chinese teachers during COVID-19 school lockdown.