So, a couple of things: I'm not familiar with OneLogin, but recently did an integration with PingID fed and ran into a couple of things not documented.
1. You have to send the X.509 cert in the SAML insertion.
2. Ensure that the right encoding is used in the SAML insertion also
(sorry for the heavy redaction)
If you go to Org > Administrators > SAML login history, you should be able to get the raw SAML XML