Oak leaves are perfectly safe when used terrestrially. Oak wood and bark is a definite NO in wet enclosures, but if relatively safe for some dry applications.)
I think where the confusion comes from is that Oak does contain quite a high amount of various tannins (tannic acid used in leather making being the most prevalent), very little of it is in the dead leaves.
When used in an aquatic set-up, you will get a drop in pH over time, but if you maintain good husbandry practices and are vigilant in your water chemistry testing you will find the pH drop is negligible. (I myself use oak leaves in both terrestrial and aquatic enclosures, dependent on the requirements of the animals.) In fact a few of the soft water amphibian species thrive when oak leaves are used in their water features!
One quick additional note; DO NOT use leaves, wood, or bark from the tree commonly called Live Oak from the west coast of the USA.
As for magnolia, I urge caution here. The name Magnolia applies to quite a few different plants. Two of them appear to show no toxicity risk from the leaves to amphibians or anything else. The rest are bad news it appears. I am having trouble making up a proper list as the common names vary tremendously and many are not clearly linked to proper scientific nomenclature. Magnolia grandiflora appears to be perfectly harmless though.
Now that you all have me thinking about it, I think I may have research this topic in depth.
