The aims of this mixed methods study were to gain insight into how individual assessors determine an Objective Structured Clinical Assessment (OSCA) result for undergraduate nursing students and identify whether individual assessor perceptions and professional characteristics have an impact on students' results. Results from 25 participants showed that although less than half (44%) of the participants were teaching in the course that they were assessing, the participants were highly experienced clinicians and nearly three-quarters (72%) had completed formal teaching qualifications. There were wide variations in pass rates (16.7-90%) between assessors. The widest disparity was observed between assessors with and those without critical care experience (66% versus 39%), as well as assessors who were teaching the course and those who were not (68% versus 49%). Qualitative analysis revealed three dominant themes within participants' transcripts. The themes focused on determining student safety, and the use of personal perceptions and clinical experience to determine competency. Findings indicate that assessors' individual perceptions and clinical experience have the potential to influence and determine undergraduate nursing students OSCA results. Development of criteria standards and objective assessment may be enhanced by greater involvement of assessors and thorough education and training within the context of student assessments.