To the OP. If your F53 chassis (Ford) is a '98 thru '03, you have a 4R100 transmission. It is a 4 speed, automatic, of course. Since it is an automatic, it does everything it needs to do without any help from you. OD is nothing but 4th gear (why mfgs call it that is beyond me and just causes confusion).
Anyway, just forget about it. Now, if you are an old manual trans guy, then you know when you go down steep grades, dropping the trans from 4th to 3rd will slow you down and help from burning up your brakes. It's no different with an automatic, except an automatic will not downshift on down grades, only up grades. So that is the reason it is there if you want it. Same reason most auto shifters have 1,2,3, so you can downshift manually.
If you have a '04 thru '10, then you have a 5R110, which is a 5 speed automatic. Begining in 2011 all trans are 6R140. These are 6 speed automatics. Both of these trans' do not have a so called OD. They have Tow/Haul modes.
Tow haul mode, changes the shift strategy, and raises the line pressure, to reduce the chance of slippage, causing heat build up. It is recomended always when towing or hauling.
Unlike the older 4R100, when the trans had OD, the old version simply turned off the OD (or just dropped to 3rd). The new design, doesnt have the OD but more gear(s), and the tow haul mode switch. This just changes the perameters of the shift requirements (doesn't simply drop a gear). It holds gears longer, downshifting earlier, not unlocking the converter, on decents, down long hills reducing brake fade. It's just a smarter more automatic transmission designed for heavy duty trucks.
John & Clare - Buck & Zeus - Jack & Husky
2008 Fleetwood Bounder 38P
Towd: 2 HD Streetglides