 No you didnt have to disengage the FWA to make a turn. The system was driven off the crankshaft of the engine, there was a shaft that went through the big white grill out to a pump and switch assembly that bolted on where you would normally have the weights. From there it had hoses that went out to the wheels and on the inside of the front tires it had a big hydraulic motor that the pump and hoses fed. The hydraulic motor is what drove the wheels, there was one on each side. I assume there was a series of gears behind the cover that took the power of the hydraulic motor and put it to the wheel. The entire FWA system was completly seprate from the rest of the tractor, in that it operated on its own without any help from the tractors regular hydraulics, or gear drive system, so really the operator was operating both drive systems. The system worked very much like a rear wheel assist on a combine only the combines are hydrostatic on all 4 wheels and the tractors were gear drive on the back and hydrostatic on the front. The pump assembly set out probably 2 feet or so and the only connection to the wheels was the hoses that ran out to them, so you could turn the tractor really sharp, that was the big selling point. The steering was the same as you would find on any wide front Oliver with the steering motor under the radiator. The hardest part of the entire system was getting the power of the front wheels to match the speed and power of the rear ones. There was a cable operated lever that bolted on under the throddle on the tractor that selected the gear that you wanted to run for the front wheels and you had to run in direct drive all the time with the front end engaged. There was electric switches that disengaged the system if you hit the clutch or if you put the tractor in reverse but they didnt always work correctly and the front end would sometimes pull when it should not. All in all it was a neat system, just too complex to work well and it only generated about 10 to 15 extra HP on the ground for all that. The other big problem was that Oliver was trying to mate two different drive systems and that was never going to work, Had the tractor been hydrostatic and the front end been hydrostatic and then they could have been connected it may have worked better. Hope this helps, and here is a 1950 Oliver FWA that was at a show.
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