Import all the users in specific OU To Distribution list

Dynamic Distribution Group is the way to go on this one, but be aware that with 1500 users there will probably be a noticeable delay in mail delivery when sending to the group, since DDGs have to determine membership at time of delivery for each message sent.

If that delay is not acceptable, it's possible to add all of the users in an OU to a specific group by using the -organizationalunit switch of the get-mailbox cmdlet, then piping that to add-distributiongroupmember, like this:

get-mailbox -organizationalunit <OU Name> | add-distributiongroupmember <Distribution group name

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I'm also assuming Exchange.

Anything we can automate when it comes to "every user" or every user in an OU, use Exchange CmdletExtensionAgent (If Exchange 2010+).  This is basically a powershell script that runs every time a user action is performed, such as a new mailbox.  

There is a file named ScriptingAgentConfig.xml.sample that is located under C:\Program Files\Microsoft\Exchange Server\V14\Bin\CmdletExtensionAgents.  Remove the .SAMPLE extension and then edit the file to do what you want.  Lots of examples online.  Here's one we use for adding every user to a retention policy when we create a mailbox -

<?xml version="1.0" encoding="utf-8" ?>
<Configuration version="1.0">
    <Feature Name="MailboxProvisioning" Cmdlets="new-mailbox,enable-mailbox">
 
         <ApiCall Name="OnComplete">
              if($succeeded) {
                Set-Mailbox $provisioningHandler.UserSpecifiedParameters["Alias"] -RetentionPolicy "_EnerTech Default Retention Policy"
               }
         </ApiCall>
     </Feature>
</Configuration>

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Once you have the file doing what you want (You'll want to add in an IF to check the OU), then you enable the agent by entering the following in PS -

Enable-CmdletExtensionAgent "Scripting Agent"

Also if you have more than one Exchange box, make sure to copy the XML to all boxes.