Undergraduate students benefit from observation of each other's oral presentations through both exposure to content and observation of presentation style. In order to increase the engagement and reflection of final year students in an oral presentation task, a peer assessment component was introduced using a rubric that emphasised scientific skills over presentation quality. This study investigated the effect of peer assessment on students' reported motivation and reflection, and their level of acceptance of peer evaluation of an oral presentation. As a result of peer assessment, students reported being more engaged, feeling a sense of responsibility, and many felt that they reflected more on their own talk. Students considered presentation style over scientific quality and demonstrated a strong reticence to award low marks. The impact on the final marks was mitigated by using a 20% weighting on the peer assessment, a level that the majority of students considered acceptable. This analysis suggests that peer assessment can achieve the intended learning outcomes. This paper provides a suggested process for using peer assessment in oral presentations with a strong science component and discusses approaches to examine and mitigate the observed student reticence to award low marks.