For omnidirectional videos (ODVs), the existing o-line coding approaches are designed based on the spatial or perceptual distortion in a whole ODV frame, ignoring the fact that subjects can only access viewports. To improve the subjective quality inside the viewports, this paper proposes an o-line viewport-adaptive rate control (RC) approach for ODVs in high eciency video coding (HEVC) framework. Specically, we predict the viewport candidates with importance weights and develop a viewport saliency detection model. Then, the predicted candidates and detected saliency are taken into account in our viewport-adaptive CTU traversal and bit allocation scheme. Finally, the experimental results validate that our approach is effective in saving bit-rates and improving subjective quality for encoding ODVs; meanwhile, our approach is also effective in the auxiliary task of saliency detection in viewports.