The purposes of this study were to investigate the effects of task-based instruction using online language games in a flipped learning environment (TGF) in developing the English oral communication ability of Thai secondary students and examine the students' opinions of the task-based instruction using online language games in a flipped learning environment. The present study employed a mixed-method approach. The two-group pre-test and post-test design was used. The participants were 80 students studying in Mathayomsuksa 3 (grade 9) at a secondary school in Maha Sarakham province in Thailand. Forty students were in the experimental group where the TGF was given as the treatment, and 40 students were in the control group where all instruction was taught only in class. Pre- and post-tests were used to collect the quantitative data. A semi-structured interview was used to collect the qualitative data from the students' opinions. The findings revealed that the students in the experimental group outperformed those in the control group after the TGF. The different mean score of the experimental group was statistically significantly higher than the control group (p < 0.05), demonstrating that the TGF was effective in improving the students' English oral communication ability. It appears that combining task-based language instruction, flipped learning, and game-based language learning could help the students improve their oral communication skills in English.