I just mean that first you need to figure out how your engine is going to work -- what actual code you are going to write -- then go about organizing that code into an object model.
A physics engine will have classes for the things it cares about like masses, constraints, collisions, contacts, etc. A renderer could have things like meshes, materials, particle systems, and lights. Each subsystem has a different idea of what a particular vehicle, weapon, or monster is. Two monsters might have the same physics properties but be rendered very differently. A Koopa and a Flying Koopa might look the same but behave differently. You can't start with classes for vehicles, weapons and monsters, and expect these classes to be used across all subsystems.
OOP is really a way to organize your code. OOP doesn't generate solutions.
A physics engine will have classes for the things it cares about like masses, constraints, collisions, contacts, etc. A renderer could have things like meshes, materials, particle systems, and lights. Each subsystem has a different idea of what a particular vehicle, weapon, or monster is. Two monsters might have the same physics properties but be rendered very differently. A Koopa and a Flying Koopa might look the same but behave differently. You can't start with classes for vehicles, weapons and monsters, and expect these classes to be used across all subsystems.
OOP is really a way to organize your code. OOP doesn't generate solutions.