We study the representation theory of the subregular W-algebra $\mathcal{ W}^k(\mathfrak{ so}_{ 2n+1},f_{ sub})$ of type B and the principal W-superalgebra $\mathcal{ W}^\ell(\mathfrak{ osp}_{ 2|2n})$, which are related by an orthosymplectic analogue of Feigin-Semikhatov duality in type A. We establish a block-wise equivalence of weight modules over the W-superalgebras by using the relative semi-infinite cohomology functor and spectral flow twists, which generalizes the result of Feigin-Semikhatov-Tipunin for the N=2 superconformal algebra. In particular, the correspondence of Wakimoto type free field representations is obtained. When the level of the subregular W-algebra is exceptional, we classify the simple modules over the simple quotients $\mathcal{ W}_k(\mathfrak{ so}_{ 2n+1},f_{ sub})$ and $\mathcal{ W}_\ell(\mathfrak{ osp}_{ 2|2n})$ and derive the character formulae.
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