Cholesteric liquid crystals (Ch’s) are formed by organic molecules that pack in space into periodic helical structure and are capable to selectively reflect light in the visible part of the spectrum when the period of the helix is in the submicron range. This property is attractive for designing optical elements such as band-pass filters, mirrors, low-threshold lasers, etc. However, the Ch period is hard to tune continuously by electric or magnetic fields. In chiral mixtures of flexible dimers, an applied field can create an oblique helicoidal structure (ChOH) which is stable in a wide range of applied field maintaining its axis parallel to the field (Xiang, J. et.al, Phys. Rev. Lett. 112, 217801 (2014); Xiang, J. et.al, Adv. Mater. 27, 3014 (2015)). Both the ChOH period and the conical angle depend strongly on the field, which enables electrically tunable Bragg reflection in a broad spectral range from ultraviolet to infrared. We explore the Bragg diffraction at the ChOH periodic structure as a function of the electric field, surface anchoring, angle of light incidence, and polarization. In ChOH mixtures doped with azobenzene-based photosensitive compound, we show that the ChOH period can be tuned by UV irradiation because of trans-to-cis isomerization. Finally, Bragg reflection at the ChOH periodic structure can be used to measure elastic constants of Ch phase. The work is supported by NSF grant ECCS-1906104.