The @OctoPrint Control tab has buttons for turning fan on/off but pressing them does not do anything. I have the Prusa i3 machine. http://shop.prusa3d.com/en/3d-printers/59-original-prusa-i3-kit-with-lcd.html
They sent M106 and M107 from memory. I’d check your firmware has the appropriate setup to turn the fan power on and off for those commands.
sounds like a plan. How do I do that ? 
I’d start by matching your power connections on your board with those in the firmware. Marlin I’d assume. You could simply see which power output was affected by the fan commands and ensure you hook up to that connector on the Ramps board.
Oh it’s rambo. Not used that at all but it should be similar to work out what’s going on.
You could also try to manually enter the g codes.
Do you know what firmware is on the controller board? I don’t know Octoprint, but maybe the default assumes a different g code than what the firmware uses?