Teachers' noticing of students' mathematical thinking plays an important role in supporting student learning. Yet little is known about how online professional development (PD)--a growing setting for PD in the USA--can cultivate this noticing. Here, we explore the potential of using two online tools for engaging video to support K-2 teachers' noticing of students' mathematical thinking in the context of a six-week online video-based PD program. While participating in the program, teachers used a "commenting tool" that allowed them to view video in its entirety and write a summary note, as well as a "tagging tool" that allowed them to mark moments of video while viewing and associate notes with those moments. We found that teachers' regular use of the "tagging tool" promoted their increased noticing of students' mathematical thinking in video. Further, the "tagging tool" and "commenting tool" appeared to function in complementary ways to support teachers' noticing. These findings contribute to the field's growing understanding of how technological advances can support the development and study of mathematics teacher noticing, with implications for the design of online teacher learning environments.