Omnidirectional video (ODV) streaming has become widespread. Since the data size of ODV is extremely large, tilebased streaming has been developed to compress the data. In this coding technology, high-quality tiles encoded at a higher bitrate for the users’ viewing direction and low-quality tiles encoded at a lower bitrate for the whole environment are sent to the client, and a player decodes these tiles. As a result, quality degrades due to coding, streaming, and the client’s buffer. Therefore, to provide high-quality tile-based ODV streaming services, qualityof- experience needs to be monitored by comprehensively evaluating the quality degradations. By taking into account the quality degradation due to the low-quality tiles, the ITU-T Recommendation P.1203 model, which can be used for monitoring the quality of 2D video streaming services, is extended to tile-based ODV streaming services. Our model is demonstrated to estimate quality with sufficiently high accuracy.