Purpose: This paper presents emerging findings from an ongoing research project which aimed to explore online lesson study (OLS) as a vehicle for teacher collaborative professional learning. Design/methodology/approach: Two parallel OLS cycles with two OLS teams were facilitated by the author using Zoom and Google Drive as digital collaborative tools. Each OLS team comprised three primary teachers who taught in three different schools, with both teams' research lessons taking cross-curricular science, technology, engineering and maths (STEM) focus. In order to explore the influence of OLS on teachers' collaborative professional learning outcomes in STEM, a qualitative case study approach was adopted, with data drawn and thematically analysed from OLS meeting transcripts, semi-structured interviews with teachers and the author's reflective diary. Boundary crossing is used as a theoretical lens to ascertain the potential of OLS as a vehicle for teacher collaborative professional learning. Findings: Findings suggest that OLS facilitated collaborative learning and positively contributed to teacher participants' co-construction of knowledge in relation to STEM teaching approaches. Originality/value: The study described in this paper represents the first OLS conducted in the Irish context and also represents the first inter-school lesson study (LS) conducted in the Irish primary context.