OK. In Angle mode, you can turn on the OA with the switch. Bear in mind the center position is not used at this time. When OA goes active you will get a vibration, and the screen icon will become green. On the Pro, OA is the RealSense. In Angle mode, RS on, as long as the H is moving forward it will sense obstacles, the ST-16 will start "pinging" and refuse to fly into them. It will stop and refuse to move forward. You then have to manually steer away from the obstacle. It cannot sense sideways or backwards in Angle mode.
Only in Smart mode, Watch or Follow, will it take control, always face farward and fly around obstacles without help. In RTH mode, you can also switch RS on.
RS will not work in Cable cam or other Special modes at this time. Remember, even with the switch on, RS may not activate for various reasons, so make sure that icon is green. Turn that switch off when landing, turn it on only in the air.
Hope this is not confusing.