The Osserain bridge is a limestone masonry structure comprising three low arches, each with a 14-meter span. Constructed in the 1880s in the Pyrénées-Atlantiques region of southern France, it crosses the Saison River. Having been decommissioned and replaced by a nearby concrete bridge for several decades, the Osserain bridge was ultimately slated for demolition. However, before this could occur, Local Authorities offered the bridge to the national collaborative research program DOLMEN, providing a rare opportunity to carry out a full-scale experimental campaign on a real, aging masonry structure. As part of the DOLMEN initiative, the bridge was subjected to an intensive instrumentation and monitoring effort in preparation for a full-scale loading test—conducted by stacking concrete blocks on a half-vault. The bridge was equipped with a wide array of sensors, to capture strain and displacement data. The research brought together 24 partners who designed and implemented a joint experimental and modeling program aimed at studying the bridge’s behavior as thoroughly as possible. This included both conventional structural analysis methods (e.g., limit analysis) and advanced numerical techniques such as Finite Element Methods (FEM) and Discrete Element Methods (DEM). This paper presents selected findings from the Osserain experiment, focusing on the comparison between experimental observations and the predictive capabilities of various modeling approaches.