Where do the oscillations on the output voltage of my buck converter come from?
Repeat your measurements but this time connect your probe tip to the same point as your earth clip. What do you see?

If your probe is picking up a signal directly due to the inductive loop it forms then you will also see it when probe tip is connecting directly to where your earth clip is placed.


  Thanks for the suggestion Andy. I wrapped a little wire around the
  ground of my probe. Then, I ran the same test. The oscillations
  disappeared ! I feel so dumb right now not to have done this
  earlier... Anyway, my SMPS works correctly at least