5.3.2.19 Various problems in the space

Condition: In the tetrahedron \( O A B C \) the point \( K- \) is the midpoint of \( O A \), \( S \) is the point of intersection of the medians of the triangle \( A B C \). Taking \( O A, O B, O C \) as the basis vectors, find \( O S \) in this basis.