Is RSA-OAEP secure against Shor's factoring algorithm
If so, then make the seed &gt;128-bit and be safe from Shor. Then, why there are so much fear of Shor's algorithm for RSA?

Well, remember that the assumption in the original question was that we kept the RSA public key secret.  That is a nonstandard assumption; we generally assume that the attacker knows the RSA public key (and in practice with most uses of RSA, he can easily learn it).
And, with knowledge of the RSA public key, Shor's algorithm can efficiently break it (assuming a reasonable sized Quantum Computer, of course); that is, he can learn the private key (and then do anything that the legitimate owner of the RSA private key can do) - the RSA padding mechanism does not offer any additional protection.