For conducting the survey, you will need an Ouster lidar, GNSS Receiver and an on-board computer, when operating in RTK mode. Whilst the lidar will create a 3D point cloud, the RTK receiver will be proving geospatial position and an on-board computer will be registering sensor readings, which are being transferred via Ethernet communication port. You can then process the GNSS and IMU data in a software like Inertial Explorer. However, the primary difficulty with integration in such way is synchronizing the timing of the different devices as well as RTK in a city can be a bit of a challenge. You can find more information about time synchronization in OusterGuide (Chapter 5). In this case, more favorable solution would be PPK mode.
Therefore, I would like to ask for what purposes you wish to integrate Ouster lidar with external GNSS? Once I will know that, I will suggest a more detailed workaround.
Looking forward to your response!