OOTX or Omnidirectional Optical Transmitter
SteamVR Tracking base stations do not generate 'perfect' laser sweeps. Each base station has its own set of eccentricities or non-ideal behavior. At the factory, these non-ideal parameters are calculated and stored in the base station. In the field, each base station transmits its calibration parameters using the LED blink-sync. During the sync period a bit of data can be sent per frame. Since it uses the optical blink sync this transmission is referred to as Omindirectional Optical Transmitter or OOTX. Valve does not publish a specification for OOTX.
Only the unofficial documentation is available for this.
Note: OOTX is part of SteamVR Tracking 1.0. SteamVR Tracking 2.0 will use Sync-on-Beam and the base station calibration data will no longer be transmitted via OOTX but will be provided by some other means.