The below is a little example of how to list all mailbox in Office 365 that have an Out of Office or an auto reply set.

 

Connecting to Office 365

$UserCredential = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
Import-PSSession $Session

Option 1 : List just the identities that have an Out of Office / Auto Reply Set

Get-Mailbox | Get-MailboxAutoReplyConfiguration | Where-Object { $_.AutoReplyState -eq "scheduled" } | fl identity

Option 2 : Getting the full details including the message and start and end times

Get-Mailbox | Get-MailboxAutoReplyConfiguration | Where-Object { $_.AutoReplyState -eq "scheduled" }