OnePlus have just officially announced the launch dates for both CM 12S and OxygenOS. According to the announcement, OxygenOS will be the first to arrive and will be available on (or before) March 27th. This will not be an OTA rollout and instead will be a download and install rollout. In terms of CM 12S, this will be available on (or before) March 30th and will come as a standard OTA to OnePlus One devices.
CM 12S: The official OTA update for the OnePlus One will be released on or before March 30, 2015
OxygenOS: The official release of our own Lollipop-based ROM will be available for download on or before March 27, 2015