Hi,@Iniobong Nkanga
Thanks for posting your question in the Microsoft Q&A forum.
You want to know how over 150 On-Prem mailboxes can be migrated to Online.
I have a couple of questions to confirm:
- Is your mailbox server an Exchange Sever?
- What is your version of Exchange Sever?
- Do you still want to keep local AD? If yes, you can only do a hybrid migration.
For Exchange Sever migration methods you can refer to this URL: Decide on a migration path in Exchange Online | Microsoft Learn
For migration assistant you can refer to this URL:Microsoft Exchange email setup guides | Configure Microsoft Exchange (cloud.microsoft)
If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.