I submitted an article to one of the OMICS journals and also paid for it only to find out they are a predatory journal. article not yet publish. Any advice what I should do?
It can't hurt to try to withdraw the article and get your money back. You can send an e-mail and request this, but they may not agree. Once you have signed the publication agreement and paid, they are legally allowed to publish the article even if you have changed your mind. I can't predict the likelihood of success, but you can't make the situation worse by asking.
You might want to withdraw the submission even if you don't get your money back, to avoid having it appear in a place that could damage your reputation. If you ask them before publication, then they ought to agree to this. (They aren't legally required to, but I don't see why they wouldn't if they can keep the money.) Of course I wouldn't request this until you've found out whether they will refund some or all of the money. Leaving them with the money would be frustrating, but you may not be able to get it back, and a publication in a predatory journal won't help you. It's better to be able to submit it somewhere else.
After publication, they shouldn't just remove the paper from the academic record. You could ask them to retract it (which would leave a retraction record), but they might not agree to this if there's no problem with the paper, and it would be embarrassing in any case. Once the paper has been published, you best bet may be to accept it and move on.