OnlyFans content is stolen using screen capturing tools all the time, and it often ends up on sites like Twitter and Reddit.
You should use the watermark built into the OnlyFans uploader, so if you see your content on a third-party site, simply ask the OnlyFans legal team to send a copyright notice to any site that’s hosting your watermarked content without your approval. But OnlyFans is a massive website, and it can frequently take weeks or even months for the team to issue copyright notices for specific content.
If you know which user leaked your content, you can block them and notify OnlyFans as well. Different countries and localities have laws about uploading sexual content without consent, and if you know the identity of the content thief, you may be able to file a suit against them (although it can be very difficult if the thief isn’t in the same country as you).