I disagree with almost all of you on one level or another.
See, here's the thing. I know for a
FACT that it is not only possible to learn ouchi gari (or any other martial arts "technique") but to also create it whole-cloth from thin air.
How do I know this for a fact? Because someone, some time in the past, did so. Mankind wasn't born with an innate instinct for doing ouchi gari. Someone along the line had to develop it. And it didn't take a martial arts genius, a fighting einstein, just a special set of circumstances. It's just a physical skill for cry'n out loud. It ain't magic.
Now, here where I agree with you. While it is possible to learn without instruction, or even create anew, a martial arts "technique" it is the
LEAST efficient of all possible methods. Even a talented athlete will have a much longer learning curve than if there was an instructor. If the athlete has a solid base in another, similar, physical activity ("tumbling" maybe?) then that can facilitate gaining a skill but it's still slower than if being guided by someone who already knows how. Further, the DIY method is simply a lot more dangerous. The "invent it myself" originator of the techniques didn't necessarily know what would make the technique dangerous to himself, leave himself open for counters, or even just put his own body in a structurally vulnerable position. A competent instructor will already know all of these things. And this goes double for weapons techniques, which also required a Patient Zero. Screw up ouchi gari and maybe you get dumped on your butt. Screw up passata sotto and you die.
But that's the great thing about the vast majority of martial arts: There are actual instructors ready and easily available to grant instruction. Even "dead" martial arts such as European Longsword has experienced martial artists working hard to bring it to functionality.
Peace favor your sword,
Kirk