Currently Lync does not support coexistence between an on-premises Lync Server and Lync Online with a single SIP domain.
This means that you cannot deploy some users in Lync Online and some users on-premises
You need to deploy Lync Server in your on-premises environment if you want to deploy Lync to both Office 365 users and on-premises users.
You will then need to decide which users will be homed to your Lync Online implementation and which to your on-premises Lync Server, based on the features required by individual users.