The correct way to define RSA_PKCS_OAEP_PARAMS is this one:
CK_RSA_PKCS_OAEP_PARAMS oaepParams = { CKM_SHA_1, CKG_MGF1_SHA1, 1, NULL_PTR, 0 };
that converted in C# corresponde to:
var mechanismParams = new CK_RSA_PKCS_OAEP_PARAMS
{
HashAlg = Mechanism.CKM_SHA_1,
Mgf = Mechanism.CKG_MGF1_SHA1,
Source = 1,
SourceData = IntPtr.Zero,
SourceDataLen = 0,
};
Let's know if it works.