Efforts to improve classroom language practices and children's language learning, as a means of supporting reading comprehension, may depend on teachers' knowledge about language structures and language development. To date, however, educational researchers and teacher educators have little understanding about teachers' knowledge about oral language. The current study reports on the development of the Teachers' Content Knowledge of Oral Language Survey (TCKOLS), a measure designed to assess preservice and in-service early childhood teachers' knowledge in this domain. The research team created the TCKOLS following a rigorous, iterative development process and conducted an initial pilot study with 32 preservice early childhood teachers and 26 pre-professional speech-language pathologists (SLPs). Findings support internal consistency and provide validity evidence for the measure. On average, preservice early childhood teachers correctly answered 69% of items and were particularly knowledgeable about vocabulary skills and development. As hypothesized, preservice early childhood teachers were less knowledgeable about oral language than pre-professional SLPs, and TCKOLS scores were significantly correlated with existing measures of vocabulary and morphological knowledge. Preservice teachers' preparation to support children's language learning, however, was not associated with their TCKOLS scores. The measure may be used in future research, to examine teachers' knowledge about language across samples and contexts and as related to classroom practices and children's learning, and to inform teacher preparation programs and professional development to better support such knowledge.