Differentiated second/foreign language (L2) instruction (DI) has received growing attention in pedagogical research. Yet, as a cornerstone of inclusive education, the DI is still under-documented. In this study, we integrated diagnostic assessment (DIA) and collaborative language learning in a differentiated (tiered) oral tasks intervention and mediated them in a Google Meet cyber classroom. To do so, an intact group of 64 non-English major university students was selected, pretested, and randomly assigned to collaborative (CG) and individual (IG) groups. Each group was partitioned into higher, middle, and lower language ability tiers. The objectives were to examine (1) how differently mixed-ability English-as-a-foreign-language (EFL) learners could improve their collaborative and individual oral tiered task performances on which they received e-DIA for 10 weeks, and (2) how differently e-DIA could affect their postintervention learning achievement in oral skills. The statistical analysis indicated the positive and equal impact of e-DIA on CG-tiers, but a positive and divisive impact on IG-tiers. Responded to e-DIA, both groups had significant pretest-to-postest learning achievement in oral skills, but CG-tiers' collaboration resonated with the impact of e-DIA to outperform the IG-tiers on the posttest. The findings implied the integration of e-DIA and collaborative learning in the DI framework to mitigate L2 learners' diversity in inclusive L2 education.