I had a CentOS 7 machine which I converted to use OEL 7 some time back using the centos2ol.sh script.
Today, I gave the command: "dnf upgrade" and it showed be that 17 packages were to be upgraded with 220MB to download.
I then selected 'N' to abort and gave the command: "yum update". This shows me that 2 packages will be installed and 46 updated with a 542MB download.
Why this difference?