There is an increasing emphasis on supporting the oral language needs of children in the classroom. A variety of different measures have been developed to assist this process but few have been derived systematically from the available research evidence. A Communication Supporting Classrooms Observation Tool (CsC Observation Tool) for children aged 4-7 years (that is, in Reception and Key Stage 1 classrooms) was devised following a review of the research literature. The evidence derived from 62 research papers was rated based on the studies' research design following specific rating criteria. Based on the review of the literature and rating of the evidence, three main areas were identified and included as dimensions in the CsC Observation Tool: "Language Learning Environment," "Language Learning Opportunities" and "Language Learning Interactions." A feasibility study was carried out in 101 classrooms in 39 schools across 10 local authorities in England. The results suggested that the CsC Observation Tool discriminated well between different age groups within the sample and between different schools. In addition, significant differences were found across the three dimensions of the CsC Observation Tool. For all year groups, scores for the "Language Learning Environment" dimension were significantly higher than scores for "Language Learning Interactions," and scores for the "Language Learning Interactions" dimension were significantly higher than those for the dimension of "Language Learning Opportunities." The study provided evidence for using the CsC Observation Tool in schools to support training and development. The tool has the potential to be used as a key feature in universal intervention studies to promote oral language in the classroom.