Quasi-one-dimensional open-shell ladder oligomers composed of fully conjugated five-and six-membered rings are investigated to describe their open-shell singlet character and their third-order nonlinear optical (NLO) responses using density functional theory calculations. The nature of the repeating structures is shown to modify the multi-radical nature described by multiple diradical characters, the bond-length alternation, and the local aromaticity. The calculation of the third-order NLO properties (γ, the second hyperpolarizability) reveals that the open-shell ladder oligomers with small multiple diradical characters exhibit the largest γ values. At the infinite chain length limit, the static γ value per fused-ring is estimated to reach about 3.6 times that of polyacene, and about half that of all-trans-polyacetylene. The present study gives the first detailed clarification of the multi-radical character and the local aromaticity in real multi-radical ladder oligomers, and the first demonstration that these oligomers are efficient NLO candidates with both large γ and high rigidity.