When healthy, Ostrea edulis populations are capable of creating remarkable biogenic reefs, providing a unique habitat for marine biodiversity. At present, the biotic assemblages associated with flat oysters remain poorly defined. This study aims to analyze biodiversity associated with the reef's development stage by focusing on its epibiotic community. The studied oyster population is an old remnant bed located in the Bay of Brest, which has been undergoing restoration for 5 years. The epibiotic communities (>500 μm) of scattered individual living (n = 10) and dead (n = 9) oysters, as well as of aggregates from the ground (n = 10) and from a restored reef (n = 3) were compared. 137 species associated with oysters were found, among which 22 were specific to individual oysters and 55 to aggregates and reefs. Although reef samples formed a distinct group in the SIMPER analysis, Shannon's (between 2.25 and 2.52) and Simpson's (between 0.84 and 0.86) diversity indices remained constant for each reef development stage. Piélou's indices (between 0.71 and 0.80) were significantly lower for aggregates displaying a less evenly distributed community. Mean abundance and species richness per sample were higher for aggregated structures. However, when standardized to abundance per centimeter square of shell surface, both parameters were significantly higher for individual oysters. Therefore, while reefs may support a higher species richness at a larger spatial scale, at a smaller scale, single oysters have a higher species richness per unit area of shell. Seemingly, despite the poor state of their remnant populations, flat oysters are still hosting important macrofaunal biodiversity.