The OSD (on-screen display) is a small electronic component that gets installed between the fpv camera and the video transmitter. Its function is to display additional information, by overlaying it into the camera’s picture. These information are some essentials for flying, such as height, speed, heading, voltage, power consumed and many more.
Some OSDs are able to display even an artificial horizon, GPS position, RF signal strength and also some in-flight data, like total distance flown and similar.
The usage of an OSD sometime help, but in some circumstances, does not do so. For a beginner, it can provide useful data like battery usage or an artificial horizon, so he can orientate better until he gets the feel of the FPV flying. On the other side, by concentrating on the OSD data, he can loose focus on flying itself, causing crash and damage.
Such models perform rather slow flying.
For the fast flying copters, the OSD does not add much value and it can be left out completely. Thus, we save power and weight.