I can't understand how the "cryptographic functions" are to be selected RSA + OAEP which are used in OAEP. How to choose these "cryptographic functions"(G and H)?
Since this picture is taken from wikipedia, I suggest reading the text beside that picture:
I think you're asking how OAEP and RSA actually are combined, and it goes like this:
For decryption, reverse the process.
To answer your question: The cryptographic functions G and H both are typically the function mgf1 (mask generating function) with SHA1 as defined by RFC 3447.
Pseudocode for mgf1 with SHA1:
I still strongly suggest you read the PKCS#1. On page 19 you can find a complete graphical representation of OAEP.