Can you map the Okta user ID as an Office 365 Immutable ID?
Solution: Not possible, as Office 365 requires an Immutable ID extracted from either On-Prem AD or Azure AD

Once brought into Okta, LDAP roles are represented as:
Solution: Email lists

When you are trying to federate (via WS-FED) Office 365 with Okta:
Solution: You can try to federate multiple Office 365 custom domains into a single Okta Office 365 app instance via SWA SSO protocol

Provisioning actions between cloud-based apps / on-premises apps and Okta are completed by using:
Solution: The OAuth 2.0 standard

In order for SAML to work, there is a need of an IDP and an SP and we know that already, but why is it so? Because:
Solution: An SP sends SAML assertions, while the IDP receives and validates them