We use Oracle as our HR system and I would like to sync some Employee data with Azure Active Directory - for example the employee id. Are there fields available for this?
You can use directory schema extensions via the Graph API: https://docs.microsoft.com/en-us/graph/extensibility-overview
Or you can do it via the AAD Connect tool: https://docs.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-sync-feature-directory...
Another option utilized by organizations that sync from on-premises AD and have the Exchange schema extensions is to use the customattributeXX attributes.