The Urysohn sphere is oscillation stable

We solve the oscillation stability problem for the Urysohn sphere, an analog of the distortion problem for the Hilbert space in the context of the Urysohn universal metric space. This is achieved by solving a purely combinatorial problem involving a family of countable homogeneous metric spaces with finitely many distances. ; Comment: 21 pages, 1 figure. This is the post-last version of the paper. A mistake had to be fixed in the article by Lopez-Abad and Nguyen Van Thé quoted in appendix. Modifications were done accordingly