Text generation--the mental translation of ideas into language at word, sentence, and discourse levels--involves oral language abilities. However, oral language skills are rarely a target of writing interventions. We ran an intervention to improve fifth and 10th graders' written production through the development of oral sentence generation (grammatical and syntactic) skills. One hundred and fifteen students--68 fifth graders (four classrooms) and 47 tenth graders (four classrooms)--participated in a stepped-wedge cluster-randomized controlled trial. Two fifth-grade classrooms (n = 35) and two 10th-grade classrooms (n = 20) received nine 90-min sessions (3 weeks, three sessions a week) of oral language intervention immediately after the pretest (experimental groups); the two other fifth- (n = 33) and 10th-grade classrooms (n = 27) received business-as-usual writing instruction and received a delayed oral language intervention after the posttest (waiting list group). The intervention consisted of team-based games to improve oral sentence generation and sentence reformulation skills. We assessed written sentence generation, written sentence reformulation, written text quality (macrostructure and language), and text writing fluency before (pretest) and after (posttest) the intervention and 5 weeks after the intervention (follow-up). The results showed that training on oral sentence generation skills can lead to significant gains in both sentence generation and sentence reformulation skills and text macrostructural quality. Improvement at the sentence level was, however, significant only for the younger writers (fifth graders).