RSA-OAEP versus RSA with Fujisaki-Okamoto construction
The Fujisaki-Okamoto construction requires a non-deterministic public key encryption scheme. Textbook RSA is not randomized, but deterministic, and so it cannot be applied to it to obtain CCA-security. It can be applied to schemes like Elgamal and Paillier that take randomness to create indistinguishable encryptions of the same message.