Hi @Mark Lui
Office online server is not included in any office365 subscription, you need to purchase it separately.
you can download it via the Volume Licensing Service Center. Alternatively, you can purchase an MSDN subscription. Various levels of MSDN sub provide access to Server software.
Office Online Server can be downloaded from the Volume Licensing Service Center (VLSC).
For more information about subscription, you can contact their Support Center.
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