American Mathematical Modeling Competition, which is international.
contemporary undergraduate mathematical contest in modeling
There may be others, but I don't know.
In most areas, college students' math competitions are non-math majors (there are similar non-professional competitions in physics and chemistry).
There seems to be no math Olympics in the university. As for another IMC, I haven't heard that there are universities involved in China. In fact, MCM is a major player every year. What's more, MCM competition is a modeling competition, which roughly means that a team solves a large-scale application problem in three days, which is very different from the mathematical Olympics in middle schools.