There is a lot to it, but the difference in friction between and OD/low axle ratio versus 1:1/high axle ratio is pretty negligible in the grand scheme of frictional losses.
Compare (for instance) a T10 with 2.64-1.77-1.35-1.00 ratios versus a T56 with 2.66, 1.78, 1.30, 1.00, .74, .50 ratios. They have basically the same ratios 1st-4th, but the T56 adds two overdrives. Highway cruise RPMs would be basically the same if the T10 was backed with a 2.37 rear and the T56 was backed with a 4.56 rear.
Its true that the T10 will provide a tiny bit less friction on the highway, but consider the much greater effort the T10 will have in the first couple gears getting the vehicle moving. Modern transmissions are also constant-mesh, meaning that regardless of which dogs are engaged on which shaft has less of an effect since all the gears are engaged regardless of where the shifter is.
Its more a function of selecting the transmission and axle ratios that best fit your vehicle's weight and torque curve. Most drivelines can sap as much as 20% of the power away as friction. The extra 1% that is consumed by an OD is often not something that affects efficiency on the highway.
In Automatic transmissions, there is also a big difference based on the design. A 700r4 places the OD planet at the front of the transmission. When OD is engaged, it overdrives the entire transmission. In a Dodge transmission like an A518, the OD is at the back. When OD is engaged, the rest of the transmission slows down. One would think that this difference would create a large disparity between efficiency, but it doesn't.
In short, frictional losses with OD are usually negligible compared to the overall inefficiencies of the driveline.