O/D on or off when driving down the highway

RVER wrote:
Thanks, did I do damage to drive with it off? I will leave on now that I know. NEW rig to me so getting used to it and did not know it had the on and off switch for O/D. LOTS to learn.

As others have said, you did no harm at all. I don't think there's any vehicle around that would overspeed the engine in drive (rather than overdrive) at reasonable highway speeds; locking it out is benign from the point of view of breaking things. If the transmission is hunting between overdrive and drive, it is better for its longevity to shift out of overdrive rather than have it keep changing gears, besides making for nicer driving.

Even things that at one time could damage the transmission or overrev the engine (like downshifting to low gear at too high a speed) are basically much impossible with modern computer controlled transmissions. If you move the lever to downshift and you're going to fast to do so, it will not actually downshift as requested until you slow down sufficiently. Even things like shifting into park or reverse while moving, on many transmissions at least, won't cause trouble because it won't do them until you're practically stopped. I wouldn't suggest testing these things out intentionally, of course, but it's nice to know that you're pretty well protected from any ill-advised or unintentional moves.