In this article we determine a generating set of rational invariants of minimal cardinality for the action of the orthogonal group O 3 on the space R[x, y, z] 2d of ternary forms of even degree 2d. The construction relies on two key ingredients: On one hand, the Slice Lemma allows us to reduce the problem to determining the invariants for the action on a subspace of the finite subgroup B 3 of signed permutations. On the other hand, our construction relies in a fundamental way on specific bases of harmonic polynomials. These bases provide maps with prescribed B 3 -equivariance properties. Our explicit construction of these bases should be relevant well beyond the scope of this paper. The expression of the B 3 -invariants can then be given in a compact form as the composition of two equivariant maps. Instead of providing (cumbersome) explicit expressions for the O 3 -invariants, we provide efficient algorithms for their evaluation and rewriting. We also use the constructed B 3 -invariants to determine the O 3 -orbit locus and provide an algorithm for the inverse problem of finding an element in R[x, y, z] 2d with prescribed values for its invariants. These computational issues are relevant in brain imaging.the action of O 3 on R[x, y, z] 4 is determined as a subset of a minimal generating set of polynomial invariants of the action of O 3 on the elasticity tensor in [OKA17]. There, the problem is mapped to the joint action of SL 2 (C) on binary forms of different degrees and resolved by Gordan's algorithm [GY10, Oli17] so that the invariants are given as transvectants.A generating set of rational invariants separates general orbits [PV94, Ros56] -this remains true for any group, even for non-reductive groups. Rational invariants can thus prove to be sufficient, and sometimes more relevant, in applications [HL12, HL13, HL16] and in connection with other mathematical disciplines [HK07b,Hub12]. A practical and very general algorithm to compute a generating set of these first appeared in [HK07a]; see also [DK15]. The case of the action of O 3 on R[x, y, z] 4 , a 15-dimensional space, is nonetheless not easily tractable by this algorithm. In the case of R[x, y, z] 4 , the 12 generating invariants we construct in this article are seen as being uniquely determined by their restrictions to a slice Λ 4 , which is here a 12dimensional subspace. The knowledge of these restrictions is proved to be sufficient to evaluate the invariants at any point in the space R[x, y, z] 4 . The underlying slice method is a technique used to show rationality of invariant fields [CTS07]. We demonstrate here its power for the computational aspects of Invariant Theory.