We consider the "missing label" problem for basis vectors of SU(3) representations in a basis corresponding to the group reduction SU(3) ⊃ O(3) ⊃ O(2). We prove that only two independent O(3) scalars exist in the enveloping algebra of S U (3), in addition to the obvious ones, namely the angular momentum L2 and the two SU(3) Casimir operators C (2) and C(3). Any one of these two operators (of third and fourth order in the generators) can be added to C(2), C (3), L2, and L3 to form a complete set of commuting operators. The eigenvalues of the third and fourth order scalars X(3) and X(4) are calculated analytically or numerically for many cases of physical interest. The methods developed in this article can be used to resolve a missing label problem for any semisimple group G, when reduced to any semisimple subgroup H.