Looijenga-Lunts and Verbitsky showed that the cohomology of a compact hyper-Kähler manifold X admits a natural action by the Lie algebra so(4, b 2 (X) − 2), generalizing the Hard Lefschetz decomposition for compact Kähler manifolds. In this paper, we determine the Looijenga-Lunts-Verbitsky (LLV) decomposition for all known examples of compact hyper-Kähler manifolds. As an application, we compute the Hodge numbers of the exceptional OG10 example (recovering a recent result of de Cataldo-Rapagnetta-Saccà) starting only from the knowledge of the Euler number e(X), and the vanishing of the odd cohomology of X. In a different direction, we establish the so-called Nagai's conjecture for all known examples of hyper-Kähler manifolds. More importantly, we prove that, in general, Nagai's conjecture is equivalent to a representation theoretic condition on the LLV decomposition of the cohomology H * (X). We then notice that all known examples of hyper-Kähler manifolds satisfy a stronger, more natural condition on the LLV decomposition of H * (X): the Verbitsky component is the dominant representation in the LLV decomposition of H * (X).