Hybrid deployment in Office 365 | Checklist and pre requirements

Dear Chris,

if all mailboxes and Public Folders moved to cloud the you can simply redirect OWA URL's to Exchange online OWA creating CNAME record on your public DNS so your users can use seamless access experience with the old URL.

for example if your OWA address is mail.microsoft.com then your CANME on the public will be 

Record Type: CNAME

host Name : mail

Point to Address: mail.office365.com

also please note that if your local Domain name is different then the Public Domain name then you need to have the same CNAME record under your local DNS server

hope this answer your query.

Thanks