Yes, OD means OverDrive. It should be kept on unless you are hualing a heavy load or need faster acceleration at/in any one particular time/situation. I know GM recommends that the car be left in OD mode at all times except during such times as stated above(I sold GM’s and their extra warranties). The other companies generally follow the same rules. The cares today are set up to be driven basically without thinking and set to their best performance settings for the average driver. That is why the OD. It provides the best for all driving conditions, generally. Towing etc. are not the normal use so then you have to use a little more thinking.
On the highway the overdrive provides an extra gear and thus higher speeds at lower rpm’s. That is why you had the problem on the highway. For highway use Always use the OD(except towing, which I assume you are not doing). Basically think of the OD as a 5th gear on a manual transmission. That way it will make things easier. Difference here is that you do not have to shift and you have the 5th gear available for the car to use when needed. Just leave it in OD and drive normally. Let the car do the rest, what it is supposed to do.
Some out there may say different but that was only for the very old cars that were the first to have OD put in them. All new ones are engineered to be driven continually in OD mode. It is only used when needed so do not worry. It will also improve your gas mileage if left in OD because it will shift better and at the proper shift times.
Here’s a test for you to try: Turn OD off and drive for one tank of gas. Fill er up and then try it in OD. There should be a noticable amount of difference. Also you will notice the rpm level that the engine is running at drop. The shifts will be smoother and at speeds the rpm’s will be lower.
Basically the only time I do not use OD is when I am out 4 wheeling. Then I need the higher rpm’s and sharper shifts. Other than that I leave it in OD. I’m not sure which surf you have but they all work the same. So, unless climbing steep hills, 4 wheeling, hauling a trailer, pulling another car or have one heck of a load in the vehicle just leave it in OD. It will save fuel, run better, survive longer and cost less in repairs later on.