If you enable 'Create a user note with email message and track action taken by a user ' for your campaign or message, omniMarketer will track campaign message opening.

When contact opens a message, a new note is created for a contact.

Navigate to 'omniMarketer \ Campaigns' and select 'Notes' from options dropdown next to the campaign.

Use options dropdown next the note to explore more details: customer and user it belongs to, campaign and message it is initiated with.