To each second-order ordinary differential equation $��$ on a smooth manifold $M$ a $G$-structure $P^��$ on $J^1(\mathbb{R},M)$ is associated and the Chern connection $\nabla ^��$ attached to $��$ is proved to be reducible to $P^��$; in fact, $P^��$ coincides generically with the holonomy bundle of $\nabla ^��$. The cases of unimodular and orthogonal holonomy are also dealt with. Two characterizations of the Chern connection are given: The first one in terms of the corresponding covariant derivative and the second one as the only principal connection on $P^��$ with prescribed torsion tensor field. The properties of the curvature tensor field of $\nabla ^��$ in relationship to the existence of special coordinate systems for $��$ are studied. Moreover, all the odd-degree characterictic classes on $P^��$ are seen to be exact and the usual characteristic classes induced by $\nabla ^��$ determine the Chern classes of $M$. The maximal group of automorphisms of the projection $p\colon \mathbb{R}\times M\to \mathbb{R}$ with respect to which $\nabla ^��$ has a functorial behaviour, is proved to be the group of $p$-vertical automorphisms. The notion of a differential invariant under such a group is defined and stated that second-order differential invariants factor through the curvature mapping; a structure is thus established for KCC theory.