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To further illustrate brilliant Jan's answer, here are the experimental crystallographic data for both structures; also notice that reduced form contains $\ce{[Co(CN)5O]}$ units tilted at about $45^\circ$ dihedral angle, whereas in oxidized form those are perfectly aligned:

1. Oxidized form $\ce{[(CN)5CoO2Co(CN)5]^6-}$, $d(\ce{O-O}) = \pu{1.447 Å}$, ICSD#6091 [1]:
   $\color{#909090}{\Large\bullet}~\ce{C}$; $\color{#3050F8}{\Large\bullet}~\ce{N}$; $\color{#FF0D0D}{\Large\bullet}~\ce{O}$; $\color{#F090A0}{\Large\bullet}~\ce{Co}$.

2. Reduced form $\ce{[(CN)5CoO2Co(CN)5]^5-}$, $d(\ce{O-O}) = \pu{1.243 Å}$, ICSD#4093 [2]:
   $\color{#909090}{\Large\bullet}~\ce{C}$; $\color{#3050F8}{\Large\bullet}~\ce{N}$; $\color{#FF0D0D}{\Large\bullet}~\ce{O}$; $\color{#F090A0}{\Large\bullet}~\ce{Co}$.

Bibliography

1. Fronczek, F. R.; Schaefer, W. P. Inorganica Chimica Acta 1974, 9 (Supplement C), 143–151 DOI: 10.1016/S0020-1693(00)89896-5.
2. Fronczek, F. R.; Schaefer, W. P.; Marsh, R. E. Inorg. Chem. 1975, 14 (3), 611–617 DOI: 10.1021/ic50145a035.