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The basic steps are you pair the phone with the other Bluetooth device and depending on the app and the level of control on the other device, you can either play the song from the phone through the other Bluetooth device or you can control the playback from the device.
What exactly is the other device? Bluetooth speakers? A car's hands free system?
You may get the best answers from other Optimus One (P500)
owners in the Optimus Forums.