Hi Jonhnny,
Thank you for your response.
>> So, you can make sure the computer also must be able to connect to all the other domain controllers for all the domains in your forest. <<
This is pretty interesting. We have an office in Asia, and they are running on a sub domain. The only server in the US that has a need to communicate is the Domain Controller, which connect to do AD replication. Other servers in the US does not communicate. It's also on a different sub net.
We only want to sync one OU to O365, which exist in the main AD domain in the US.
But from what you said, the DirSync will try to communicate with other DCs? Do you know if this is just for the configuration phase? Or also while it's doing the sync?
I'll try to setup a VPN access on the sync server and see how it goes.