Azuma et al. showed that a cactus-expandable Boolean network $Σ$ is structurally oscillatory if and only if all the simple cycles of $Σ$ contain an odd number of inhibiting edges. We show that we do not need the cactus-expandable condition. Namely, a strongly connected Boolean network $Σ$ is structurally oscillatory if and only if all the simple cycles of $Σ$ contain an odd number of inhibiting edges. Additionally, we provide a characterization of a structurally oscillatory Boolean network for the general case.