What is the difference between oppression and suppression?

Suppression is what Trump is doing to the truth. Oppression is what he wants to do to the country.

oppress refers to pushing down, suppress refers to refusing to allow to rise. you could think of it as the one means force into hiding whereas the other means keep hidden. The idea is the the objects of the verb start in different conditions. You cannot typically suppress that which is already released, or oppress that which is already downtrodden (buried), but it is possible to suppress something that is in the process of becoming free or widespread (like suppressing an uprising).

"Suppression" applies to something specific. Suppression of religious observances, suppression of language, suppression of freedom. It can be part of "oppression", which refers to systematic denial of rights to individuals. For example, under the USSR, religious observances and native languages were suppressed in the countries that Russia took over, like Lithuania, Estonia, Latvia, etc. People are oppressed when they are denied educational and occupational opportunities.

The relatively minor difference is oppression is putting someone/something (like an idea) down whereas suppression is preventing somebody/something from rising or being expressed/free.

When talking about another person ..
Oppression is ''putting someone down''.
Suppression is ''stopping someone from getting up''