Centos 6 went EOL on November 30th, 2020. As a result of this yum will complain about not finding a valid baseurl:
Loaded plugins: fastestmirror, security
Setting up Update Process
Determining fastest mirrors
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
Eg. Invalid release/repo/arch combination/
removing mirrorlist with no valid mirrors: /var/cache/yum/x86_64/6/base/mirrorlist.txt
Error: Cannot find a valid baseurl for repo: base
This makes it impossible to update OP5 Monitor to the latest (EL6) version. This becomes a problem when you want to migrate your installation to EL7 since our recommendation is to update to the latest version on the old server before backing up.
To get rid of these error messages and make it possible to update OP5 Monitor you need to change your Centos Base repos in /etc/yum.repos.d/CentOS-Base.repo as follows:
Comment out all mirrorlist lines, remove the comment from baseurl and change it to "vault.centos.org". This way yum will stop complaining and you will be able to use the OP5 repos.