This paper discusses the challenges of defining coherence in the context of oral language assessment literacy and proposes that better understanding of the construct can be achieved through a systemic-functional linguistic lens. Coherence is taken to be a foundational quality of written and spoken discourse and is a standard feature in the assessment rubrics of most large-scale English-language oral proficiency tests such as IELTS, TOEFL, and Cambridge. However, the precise meaning of coherence, and how it can be identified in language, is difficult to grasp. This has implications for the makers and raters of tests as well as the candidates who take them. The literature suggests coherence is the most troublesome category to rate reliably and validly. The systemic-functional linguistic (SFL) model of language provides a rich description of coherence, but has been under-utilised by the language testing industry to date. In this paper we examine three aspects of coherence: (1) how it is currently conceptualised in oral language proficiency testing; (2) how it is conceptualised in linguistics in general and SFL in particular; and (3) how insights from SFL could inform the evaluation of coherence in oral language proficiency examinations.