I've been researching and from what I've seen, both are maintained by Oracle… But if java is also open source, what's the need for OpenJDK? Or am I wrong about Java being open source?
Nothing crucial. The openjdk project is mainly based on hotspot source code donated by Sun.
There is a more detailed answer to your question here (in English), which links to this blog post:
Q: What is the difference between the source code found in the OpenJDK repository and the code you use to build the Oracle JDK?
A: It's pretty close – our build process for Oracle JDK releases is based on OpenJDK 7, adding just a couple of pieces like the deployment code, which includes the Oracle Plugin Java and WebStart Java implementation as well like some third-party closed source Like a graphics rasterizer, some open-source third-party components, like Rhino, and a few bits here and there, like additional documentation or third-party sources. Going forward, our intention is to open all Oracle JDK sources except those we consider commercial features like JRockit Mission Control (not yet available in the Oracle JDK) and replace blocked third-party components with open source alternatives to achieve closer parity Enter the code bases.