OEL is free to run, BUT, there is a subscription service you have to pay for to get any updates or support. Without the subscription OEL loses value very fast. Same with RHEL.
For desktop, it is best to stick with community distros IMO. If you like bleeding edge, then Fedora, and if you are more about a stable environment, then Debian, Ubuntu are nice.
If you want to get into a more custom and more intermediate level OS, then Slackware or Arch might be of benefit.
For servers I prefer Debian or Centos.