You need to provide the private key to putty, and the public key to OpenSSH.
So (if I have this right)
cat id_rsa.pub >> authorized_keys
and then specify the id_rsa file for putty
That said, I think that putty uses it's own format for ssh keys. I set this up using puttyen, available
wherever good puttys are downloaded.. It produces the files as putty wants them, and provides the authorized_keys text for easy pasting.