To quote from RFC 3447, §7.1:
RSAES-OAEP can operate on messages of length up to k - 2hLen - 2 octets, where hLen is the length of the output from the underlying hash function and k is the length in octets of the recipient's RSA modulus.
This is defined in terms of octets; exactly how many characters that'll equate to will depend on the characters you encode. One code point can occupy more than one octet, and one character (as most people think of it, anyway) can require more than one code point.