The objective of the current study is to compare how Thai EFL writers develop and express their oppositional ideas in arguments and to compare their use of oppositional connectors in arguments to those of published scholars in the field of health science. An investigation of thematic progression pattern was conducted to examine whether a certain connector frequently occurs in a particular type of thematic progression. Classifications of oppositional meaning categories (Izutsu, 2008) and thematic progression patterns (Danes, 1974) were incorporated as the framework of study. For the purpose of the analysis, two substantial large corpora, the Mahidol University Learner Corpus (MULC, 4.5 million words) and the Scholar Corpus of Health Science (SCHS, 2 million words) were developed by the researcher. Five hundred segments from each corpus (a total of 1,000 segments, approximately 1,000,000 words), containing oppositional connectors and thematic progression, written by 50 Thai EFL graduate students and 50 scholars in health sciences were analyzed as sample texts. Coding schemes for the analysis were validated and achieved absolute agreement between inter-raters. The British National Corpus (BNC) was used as a referent corpus in a pilot trial while the Corpus of Contemporary American English (COCA) was referenced in the actual analysis. One-way, two-way and three-way ANOVAs, and the Universidad Autonoma de Madrid (UAM) corpus tool, which provides chi-square statistics, were used for data analyses. Findings revealed that both groups of writers preferred to use concessive connectors to express their oppositional ideas and to use derived thematic progression pattern to organize their texts (ps < 0.001). Additionally, no major differences were found in the use of concessive connectors, the accuracy in how these connectors were used showed student writers did not use concessive ideas in the same way as scholars and, at times, students used them inaccurately. Important findings of differences in the use of oppositional connectors and thematic progression patterns are discussed from the perspectives of cognitive linguistics, cultural influences, and EFL academic writing teaching and instruction. The current study also provides evidence-based recommendations for EFL academic writing curriculum and instructional development. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]