An excerpt from a good article about just that topic:
"More interestingly, Opera only makes money through three revenue sources. The main one is a deal with two search engines. Yandex is the default search engine in Russia, and Google is the default search engine in the rest of the world. As the company’s user base grows, partners pay more money to remain the default search engine.
“A small number of business partners contribute a significant portion of our revenues,” the company writes in its F-1 document. “In 2017, our top two largest business partners in aggregate contributed approximately 56.1% of our operating revenue, with Google and Yandex accounting for 43.2% and 12.9% of our operating revenue, respectively.”"