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Before starting out with Oracle Linux this summer, my experience with Linux had been more or less confined to Ubuntu, which stems from Debian. Oracle Linux, on the other hand, stems from the Fedora project and Red Hat. Though they both build on the same kernel, they diverge from each other in a few important aspects. Software available in the repositories for Debian are divided into free, non-free and contrib. All software available in Fedora’s repositories are free.
To me, the single most significant differences between Debian and Fedora (and by extension between Ubuntu and Oracle Linux) is how software is distributed. Anyone who has used Ubuntu knows the apt-get command. Try that in Oracle Linux, and you get an error. Apt-get simply doesn’t exist. The reason is that Oracle Linux doesn’t use the apt-get dependency resolver, but rather one called yum. Add to that different formats (deb and rpm), and package managers (dpkg and RPM), and you start to see the level of difference. For a list of yum commands, have a look here.
There are other differences, too, I’m sure, but none are as important for basic use and understanding of operation of the OS as the ability to install software.