SSH keys (keypairs) are stored in two parts after creating an instance: a public part and a private part. Most instances, including those launched using an official OMI, require a keypair. It is very important that you save both parts because the private part is never provided to Outscale, and cannot be retrieved if lost. The public part of the keypair is, however, stored by Outscale.
For more information, see About Keypairs.