When you enable the Double Opt-In setting for the MailChimp connector, MailChimp will send a Subscription confirmation email to each new contact in each list.
Sample: Subscription Confirmation Email
Read MailChimp’s How do I customize each part of the confirmation process? article to understand how to customize this email.
Remember: even if OneSaas pushed contacts into a MailChimp list, the contacts will not be visible as subscribers in your list until after they confirmed the subscription.
If they never confirm the subscription or if the subscription request email is caught by spam filters, your contacts will never become a member of your list:

Remember that if you uncheck the Double Opt In check box and abuse of this functionality MailChimp can close your account.
Further references from the MailChimp help: