There is no single "Oracle killer." Mongo DB works fine for some apps, but Mongo is not going to be a successful public company. There is too much competition and too few customers are willing to pay a lot of money for a product that is open source with limited functionality. (Check out the reviews on Glassdoor) Once Mongo is under the public spotlight, it will all be about revenue and earnings, and they will not be able to handle the accountability. Cloudera and Hortonworks are below their IPO price despite a huge runup in the NASDAQ and continue to lose milions every quarter. Sorry, but "we lose money on every subscription we sell, but we'll make it up in volume" is not a strategy. There is going to be a BIG stock market crash for recent IPO companies that are losing money, as there quite simply are too many of such stocks on the market.
Oracle is under a serious threat from several cloud-based data management solutions. Oracle themselves have not invested enough in cloud infrastructure and are offering basically just the same old, expensive Oracle database running as a cloud service. They doubled their already high prices to run Oracle in AWS and Azure in January. The Oracle DB is the most feature-rich of any database, but it is far too expensive, and for most applications, alternatives are available.
Posted by @PvRfKUC-qnu, hits the nail right on the head.