Hi, Scumdog from New Zealand here - hope I can help you about the o'drive, I had one in my Mk2 Zephyr and it is the same principle as yours.
From memory if you pull the dash cable to put it in overdrive the o'drive will work once you are faster than a given speed, i.e. 35mph. To engage it you wind up to above that speed (with the dash cable pulled - it may be vice-versa in your case) and take yor foot off the gas pedal, the overdrive will now engage and give you long legs - some o'drives only work in 2nd and top gear.
When you want to pass a car on the freeway you mash the pedal to the floor for a split second and the o'drive drops out and you are into direct drive for acceleration. (the switch on the floor "kills" your ignition spark for a second taking the load off the motor enabling the solenoid to pull the 'box out of overdrive)- after you finished passing just take your foot off the gas and viola! you are back into o'drive!
The great thing I found with the o'drive was that you could leave it engaged 99% of the time and below about 10mph you could pull into low gear without using the clutch as the o'drive is fitted with a sprag (one-way) clutch, very handy when you have a non-synchro first gear.
The o'drive has a mechanical lock-out so it does not work in reverse or you would screw it up big-time. Hope this helps, Cheers, Scumdog