I've never done anything with OAEP, but crypto hash functions (as described in step 3) use a procedure spelled out in http://en.wikipedia.org/wiki/PBKDF. Basically, to expand the number of output bits, you 1st repeat the hash with an incremented counter concatenated to the argument being hashed, then concatenate those results until you have enough bits. This technique doesn't add entropy to the result, but does allow you to create a longer output bitstream.
From wikipedia: If you want a key that's dklen long, and your crypto hash function U only outputs hlen bits:
DK = T1 || T2 || ... || Tdklen/hlen
Ti = F(Password, Salt, Iterations, i)
F(Password, Salt, Iterations, i) = U1 ^ U2 ^ ... ^ Uc
U1 = PRF(Password, Salt || INT_msb(i))
U2 = PRF(Password, U1)
...
Uc = PRF(Password, Uc-1)
(If you only need one iteration of the cryptographic hash function, c=1, so you don't need the XOR operator ^, and for each i, you only need to calculate U1)