Oral Proficiency Interviews (OPIs) are widely used to measure speaking ability in a second or foreign language. The Michigan English Language Assessment Battery (MELAB) Speaking Test is an OPI used for academic and professional purposes around the world. However, little research on this or other OPIs has quantitatively compared test takers' speech with the target domains of the test. Such a comparison could be used as evidence for the validity argument (Kane, 2013) for the MELAB. In this study we use corpus-based register analysis and Multi-Dimensional (MD) analysis, investigating a large number of linguistic features to determine the extent to which the language of the MELAB is similar to conversational, academic, and professional spoken discourse, specifically nurse-patient interactions, since many of the test takers are preparing for nursing licensure. The results show that the MELAB has similarities with conversation in its use of stance, and is closely aligned with academic registers and nurse-patient interactions in the use of language for informational exchange, which provides support for the validity argument of the MELAB. However, the use of narrative features and discussion of future possibilities and suggestions are important aspects of both conversation and academic and professional registers but may be harder to evaluate through the MELAB and other similar OPIs.