Planning to deploy a new Active Directory On Premises

Hi,

We are managing an IT enterprise with more than 300 employees using Office365. We haven't an On-Premises AD, but we need to deploy it. All our users are created in Azure AD, but we are reading about DirSync and we understand that it only syncs in one direction, from On-Premises to Azure AD.

So... How can we deploy an On-Premises AD, connect to our Office365 AD and use all the already created accounts.

Thanks!!