The scarecrow is one complexly simple guy. He claims he needs a brain but the entire movie, he is the one with the plan, knows where to go, and thinks of ways to solve problems. He is just too trapped in the moment to realize it. The scarecrow doesn’t need a brain; he just needs some Ritalin to focus his thoughts. With his ADHD under control, the scarecrow can not only scare crows, but also actually earn the degree that the wizard just handed to him.
The lion claims to be cowardly, but in actuality he’s just narcissistic. He is unhappy with his image so at first tries to hide it in his outburst when you first meet him, and for the rest of the movie, tries to cope with his poor body image. When the wizard gives him the medal, it is exactly what he needed to show off to people, and in actuality instead of helping the lion with his condition, the wizard just makes it worse with a quick fix.
The tin man has Munchausen’s syndrome, which is obvious because of a simple chemical reason; tin inhibits rust. The tin man needs his heart to make his crying and “rusting” become more believable. The reason he dances around after being oiled up is because people are paying attention to him so he’s happy, but only when Dorothy leaves does he cry and start to rust again. The heart the wizard gives him will only make the tin man’s condition worse.
It turns out the wizard isn’t as smart as everyone thinks he is, and only seems to make problems worse.