Sellouma,
The install on a OCC couldn't be much easier. The cable length is just right. Avoid bends, lube flex cable befor installing. Don't over tighten any ty-raps if used.
If you can remove the clear lens, remove the two bottom screws and take out the movement.
Be sure that the internal gears are lubed.
Take note of the hair spring (you may have to shorten it later if your speedometer is not accurate.
If you have done the above, be sure to tape the lens on befor riding the bike(that way you can remove it later to calibrate the hair spring). When you know that it is right you can permently attach it.
I also run one of the cheap electronic speedometers.
The odometer knob can be removed by un screwing it(plug hole). I have about 1K Mi. on my analog speedometer. I have been told that the Wizzer analoug ($50 version) is exellent.