Overdrive simply means the ratio of GEARBOX input to output is a certain way, ie there is less rotation from engine going in than there is coming out and it is termed overdriven, most 5 speed gearboxes are 1:1 in 4th so same in and out speed of gearbox, but 5th gear is an overdrive so more speed coming out of gearbox than going in.
I get the impression this is NOT what you were after but answers the question as to what overdrive is.
As we are talking automatic boxes the same applies, as regards engine speed in and out of the gearbox.
Overdrive will NOT select or go IN below a certain speed as the car simply won't have the power to pull the high gearing in an automatic even with the torque convertor.
I get the impression from your description you are getting SLIP in overdrive, if it won't go over 45, it needs looking at, may just be low on TRANS fluid, ATF, so worth checking this first.
Any decent auto trans specialist can tell you what is wrong by driving the car.
tabetha