OTO is the short form given for One Time Offer. As the term suggests, one time offer is an offer that is made available only one time. And this offer could be something of real irresistible in value.
In other words, OTO is a page where you’ll create an exceptional quality offer such as a bundle of ebooks or softwares (or anything that complement / supplement your main theme / product) that you give special discount to; say $27 instead of the normal price of $97. And your visitors would have only ONE chance to get your deal for $27 instead of $97.