At longer distances, the repeater height does play a huge part in how far it can send a signal.
However a mobile is still looking at the horizon for the signal at longer distances so an antenna with a low angle response is the better choice.
The OCF antenna design being referred to is about a 1/3 wave length radiator with four 7 to 10 inch radials making it in 'effect' and OCF design same as a horizontal OCF would be.
Really it doesn't. You're building a 1/3 wave groundplane antenna.
The idea is to get the center length to match at it's best then adjust the radial angle for more SWR improvement.
How is that any different to a quarter wave groundplane antenna which has a feedpoint impedance partially dependent on the angle of the radials?
I travel 70+ miles out through the mountains from the local repeaters in the area and see where the signals are low and lost.
I can tell you that sat here. It is where the repeater antenna is masked from your antenna by the terrain around you. Firing a signal more horizontally will not overcome this. And when you're in the same valley as the repeater is on the mountain top several hundred feet above, firing out a signal horizontally is worse than one with a more vertical take off.
Out beyond these distances at high elevations I can still hear and get back into two repeaters.
You will because you have clear line of sight. At that point then an antenna with a more horizontal take off angle is beneficial.
I struggle to get wifi throughout my house yet I can put the router outside and get good signal strength accessing it with my mobile phone from several hundred feet away.
Bottom line it's all Ham radio and playing with designs when the conditions are there to do it.
Absolutely but I suggest you re-read your course study material about how VHF works. It may save you a lot of head scratching wondering why you've a notable improvement at high elevations but a degradation at lower ones.