Teachers' self-directed professional development (i.e., professional development implemented by teachers on their own) has been considered a useful channel of overcoming external constraints in under-resourced contexts, where external support for teachers may be limited. However, almost no research has focused on how such development could be achieved through open educational resources (OERs), which have been acknowledged for their free and convenient access and for their complementary role in buttressing the instructional content of in-class teaching. In response to this gap, this study adopted a qualitative case study approach that tracked a secondary teacher from an under-resourced vocational school in China and explored the trajectory of her self-directed professional development through OERs. Through qualitative analyses of data sources (e.g., the teacher's reflections, interview responses and field notes on teaching activities), this study reveals that the teacher's OER-based self-directed professional development involved a process of battling diverse factors in and outside the classroom, including her prior expectations of OERs and the vicarious teaching experiences that she gained while implementing self-directed professional development. The teacher gradually constructed her malleable and critical understanding of OERs as crucial resources in sustaining her self-directed professional development. This study concludes that OERs were useful in the process of the teacher's self-directed professional development, despite the complexity of their use.