You can use the following command to find out which packages have changed after an update. This shows the difference between two ostree images on your system:
rpm-ostree db diff --changelogs
In theory, you could also get a list of changes before the update.
To find the commit ID of the available image, use this command:
rpm-ostree upgrade --check
To find the commit ID of your current active image, use this command:
rpm-ostree status
Finally, you can see the difference between the two images with the following command:
rpm-ostree db diff [commit ID of your active image] [commit ID of the available update] --changelogs