The Android O developer preview is already available for selected Pixel and Nexus Smartphones. The final version of the Android O will get official later this year.
OnePlus has recently said that it would be killing the OnePlus 3T very soon because the OnePlus 5 launch in nearing. According to the latest leak of an internal mail leak, the OnePlus 5 may be released on June 15.
Going with the earlier leaks, the OnePlus 5 will ship with Android 7.1.1 Nougat and will likely get Android O along with the OnePlus 3/3T. That said, the OnePlus 2 and OnePlus X won't be getting Android O.