My current development project is SSHMon, a Monitoring as a Service application that can use SSH to directly monitor servers.
The SSH library in use, SSH.NET doesn't currently support OpenSSH keys, leading to the below error if I try to use OpenSSH instead of RSA format keys.
Luckily, the OpenSSH lib has a way to convert formats, and I use that to convert any OpenSSH keys into RSA format. The command below will convert id_rsa from OpenSSH into RSA format.
ssh-keygen -p -m PEM -f ./id_rsa
Hopefully this can help someone else.