Between 2005 and 2011, using manned research submersibles, we compared the fish assemblages associated with the midwater platform structures (at depths between 40 and 195 m) of two southern California oil and gas platforms, Gail and Eureka. Gail is a typical California platform, with rounded crossbeams and pilings, while the midwater jacket of Eureka, studded with bowl-shaped piling guides, is more complex. While the assemblages of both platforms were dominated by rockfishes ( Sebastes spp.), there were also significant differences. Compared to Gail, Eureka: (1) exhibited higher densities of all species combined and of most species in common, (2) had more mature individuals of most species, (3) exhibited greater species richness, and (4) had higher densities of species typical of complex high relief. We propose that the complex midwater jacket of Eureka, with its many sheltering sites, resembles rugose natural rocky reefs. This research both reinforces the conclusion that many reef species have quite specific habitat requirements and that the platform decommissioning process must consider each platform individually.