selected container can not be sync to O365 portal

After u alter ou selection you must run full sync

"start-ADSyncSyncCycle -PolicyType Initial

Once that done, from next time you can run delta sync

I ran the the 2 commands below in order, I still don't see the new user account I created in premise Ad in portal

start-ADSyncSyncCycle -PolicyType Initial
start-ADSyncSyncCycle -PolicyType delta

one more thing into O365 portal I can not find DirSync status, any idea ?

Check Azure AD sync from azure ad portal

May be you can reboot ad sync server and check again

Also check event viewer for any errors

One more thing, ensure that you have not disabled inheritance on those ad objects else they can't sync to O365
In that case enable inheritance and sync again

where to check inheritance on ad objects ?

Go to ad object properties and navigate to security tab and click advanced, there enable inheritance if disabled

If u can't view security tab, from aduc console enable advanced features view

Restarted and checked objects is enabled for inheritance , may I ask that I the portal why I can't see "dirSync status" under "health"

Did you stopped AD sync functionality from AZure AD?
In that case only it should vanish it from o365

If yes, enable functionality again
https://docs.microsoft.com/en-us/office365/enterprise/turn-off-directory-synchronization

The functionality is working previously or not worked since start?

View directory synchronization status
Sign in to the Office 365 admin center and choose DirSync Status on the home page.
Alternately, you can go to Users > Active users, and on the Active users page, choose More > Directory synchronization. On the Directory Synchronization pane, choose Go to DirSync management.

you can also use the option of add card in O365 portal and add the dirsync from the card.

the premise domain user could now sync to office 365 now, I have select all Container rather than specific container, and I still can not find DirSync status in portal, after going to active user > more >directory synchronization> directory synchronization pane

one more question, after my premise ad user account sync to Office365, i see the domain display as   keith@abc.onmicrosoft.com rather than showing keith@domain,com, is that something not right ?

what is your onpremise user userprincipalname  ?

Is it same as domain.com or it is different?

If current UPN is not registered with O365 as accepted domain, then what you see is expected

change onpremise UPN to match email domain and sync again..

Your issue is resolved or not?

My default domain in office 365 is now pointing to abc.onmicrosoft.com rather than domain.com, I need to make domain.com as default domain in order to get it fix ? And yes I have resolved the sync issue

I have total 500 mailboxes need to migrate from on premise exchange to office 365, I don't plan it to migrate them in one go, how do I do if I migrated 50 mailboxes to office365, are they still be able to receive mails ? Office365 still not yet to make my domain.com as default domain

making domain.com in O365 will enable you  to create cloud users with that new domain as default UPN

But you have on premise environment, if you place domain.com as UPN with on premise users and if same domain is verified at O365, O365 will put same domain as their UPN in O365 regardless of which domain is default in O365
U need to check on premise AD user UPN if it set correctly

U can setup full hybrid environment with onpremsie exchange and O365 and then migrate in the batches of 10 / 25 / 50 users or whatever batch size you like and can keep hybrid environment for long duration or for ever as long as your on premise Exchange org has Exchange 2010 and above servers