As one variant of the conventional flipped model, online flipped learning is increasingly implemented and it becomes important to explore how learners may be ready for it and how learner readiness may relate to motivation, attitude, and support. Informed by a multidimensional conceptualization of learner readiness and motivation and engagement, this study investigates the current situation of learner readiness, focusing on the moderating roles of learner attitude and environmental support in moderating the interrelationships between learner readiness and motivation and engagement in online flipped learning. Based on survey responses from 6,364 English learners across 11 Chinese universities, the results reveal that the students exhibited a generally high level of readiness for online flipped learning, though with significant demographic differences. The results also indicate that learner attitudes and environmental support moderated the impact of learner readiness upon learner motivation and engagement in online flipped learning. The findings call for attention to examine closely what dimensions of learner readiness could shape what aspects of motivation and engagement rather than simply assuming a linear causal relationship between readiness and motivation and engagement. A potential polarizing effect in online flipped learning is also highlighted, with implications discussed.