Since most schools and learners had no choice but to learn online during the pandemic, online learning became the mainstream learning mode rather than a substitute for traditional face-to-face learning. Given this enormous change in online learning, we conducted a systematic review of 191 of the most recent online learning studies published during the COVID-19 era. The systematic review results indicated that the themes regarding "courses and instructors" became popular during the pandemic, whereas most online learning research has focused on "learners" pre-COVID-19. Notably, the research topics "course and instructors" and "course technology" received more attention than prior to COVID-19. We found that "engagement" remained the most common research theme even after the pandemic. New research topics included parents, technology acceptance or adoption of online learning, and learners' and instructors' perceptions of online learning.