@Joel Saripelli Thank you for reaching out to us, As I understand you are trying to sync custom attributes via Okta sync tool to O365/Azure AD, however Okta sync is not allowing to sync custom attributes.
We dont have expertise to comment on the Okta sync why custom attributes are not syncing to O365/Entra ID, Okta team would be the best to comment on this.
If the custom attributes are less in number to be updated, you can explore this option - https://learn.microsoft.com/en-us/graph/api/application-post-extensionproperty?view=graph-rest-1.0&tabs=http of creating attributes manually via Graph explorer for the users or consider moving to Entra Connect/Entra cloud sync.
Reference: https://stackoverflow.com/questions/68170000/how-to-add-custom-attributes-for-azure-ad-b2b-user
Let me know if you have any further questions, feel free to post back.
Please remember to "Accept Answer" if answer helped, so that others in the community facing similar issues can easily find the solution.