A composite system consisting of a degenerate optical parametric oscillator (DOPO) and N two-level atoms interacting with a broadband squeezed vacuum (SV) centred at frequency ωs and an input monochromatic pumping field with a frequency ωp is analysed. The corresponding explicit analytical steedy-state solutions in the central mode ωp = ωs are derived, and the result displays optical bistability (OB). In addition, the influence of the broadband SV on the bistable behaviour is analysed in detail.