If your organization needs a federated identity, you will still need directory synchronization as a base requirement. You may use your ADFS servers with Office 365 if you already have them in your infrastructure.

To use federated identity, you can also create your infrastructure using Azure AD Connect custom installation. Go to https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnect-get-started-custom and skip to the section named Create a new AD FS form or use an existing AD FS form.