If your OSCar suddenly doesn't make any sound or starts behaving strangely, don't bang your head on the wall, it probably just needs battery replacement.

Before replacing the battrey, if your synth still have patches in memory, backup them on a cassette or through MIDI. Please refer to the Owner's manual. If your synth already lost all the patches, just read on.

FIRST OF ALL and VERY IMPORTANT: disconnect the synth from the plug and wait at least 10 minutes before putting your hands on it.

Remove the front plastic panel, it has no screws, just pull gently on the back, with the aid of a screwdriver or something similar as a lever. Don't pull too hard or you can break it.
To ease the operation I suggets you to loosen the screws that hold the rubber side panels. This way the front panel will be free on the sides and easier to remove.

Pay attention to the small rubber "dividers" on the front panels, they break quite easily. Also pay attention to the small knob caps, they easily jump out and can really "fly" everywhere.

The battery is the small white cylinder at the center of the main board

click on the image to enlarge

Before proceeding I suggest you to remove all the small buttons, because while moving the synth they easily "fall down"

Then remove the black back panel. The panel is fixed by means of two small aluminium bars, which are screwed on the rubber side panels. Completely unscrew the two bars. Only this way the back panel will be free to move, and you can have access to the bottom part of the PCB. Pay attention to the multi-color cable which connects the MIDI plugs to the PCB, don't pull it.

click on the image to enlarge

If you can reach the contact on the bottom of the battery you can start desoldering them. I suggest you to completely reove the main board PCB for ease of operation. It is not screwed, the two aluminum bars hold it in its position. In this case pay attention to the various cables which connects the main board to the synth power supply.

click on the image to enlarge

The battery is fixed with three soldering points, just de-solder them and remove the battery and replace it, quite simple. See above pic for correct Voltage and mAh of the original battery (I found a new one, an AIWA 3.6 Volts, 140 mAh for 7.30 Euros at my local electonics shop).
Replacing the battery will erase all the patches for the OSCar internal memory. Once the battery has been replaced, if you want to have the original sounds back again, you will have to send the synth the original sounds through MIDI. I have a MIDI file available on my site, which contains the original factory patches