First to enable the OSC Server part of Bidule, you need to go the OSC Server dialog by selecting OSC Server from the Tools menu. You need to select a port for Bidule to listen for messages (Note: you might need to change your firewall software settings to allow incoming messages to that port), the default one should be fine but you can change it.
Every parameter of every bidule that is available for parameter linking is also available for control through OSC. Each parameter has an address to receive values, the address is built automatically from the location and name of the bidule as well as the parameter name. The location of a bidule is the "path" to it (look at the bottom left of the patchbay when you're inside a group to get the idea) so for example a group named Freeverb in the root of the layout would be /Freeverb, now if we wish to control the mode of the effect, the address would be /Freeverb/Mode. Take note that some characters (like spaces) are reserved and this might change the "expected" address so your best bet is to look into the OSC Server dialog and see the name that is there.
Each parameter expect to receive values within its range (click the Params button on the bidule's window to see them). Parameters that are a drop down box with a list of choices will also accept a string representing an available choice.
These two address also exists: /play : to start/stop the processing (parameter: 1 or "true" to start audio, 0 or "false" to stop it) /open : force the loading of a .bidule file (parameter: the path to the bidule file on the machine)