Over-the-top (OTT) services are considered to be online mobile applications that are typically provided by third parties through app stores and often consume significant mobile network bandwidth. With billions of users, OTT services already have huge scale and still have further growth potential. In this article, we discuss four kinds of OTT services - consumer messaging & voice, mobile gaming, video and music streaming, focusing on the impact to telecom operators and the options to address and benefit from collaborating with OTT players.