I've installed Oracle JDK using this guide. Then I accidently installed OpenJDK(tried to prepare PC to build Android). How do I get back to Oracle Java? Can I keep OpenJDK, but set Oracle Java as default?
You should be able to keep both versions without a problem. I found a link that describes how to switch between Java versions on Ubuntu.
To switch between Java versions:
sudo update-alternatives --config java
To switch better javac (complier) versions:
sudo update-alternatives --config javac