Oppress means to persecute/tyrannize/bully. A nation could be oppressed by its leaders, a community could be oppressed by a powerful segment intent on control, and a child could be oppressed by bullies.
Further, people can feel oppressed (worried/depressed) by many things, such as a strange atmosphere or strange surroundings.
Repress means to restrain, limit, subdue or end something. People can repress a smile if they are in a serious situation where smiling isn’t appropriate, and they can repress memories, but they can rarely repress a sneeze.
On a larger scale, governments can repress a rebellion as well as repress inflation.