Oregano essential oil (OEO) was evaluated to determine its effect on the growth of natural contaminating molds on the surface of Spanish fermented sausage, the development of the internal microbial population of the sausage, and the physicochemical properties of the sausage. Results indicated a dramatic reduction in the contaminant molds. At the end of ripening, the main endogenous fungal species in control samples were Mucor racemosus (55%), Aspergillus fumigatus (20.6%), Cladosporium sphaerospermum (11.1%), Acremonium strictum (7.9%), and Aspergillus niger (4.7%). In samples treated with OEO, M. racemosus and A. fumigatus were the only species isolated; the treatment was more effective against A. fumigatus than against M. racemosus. The use of OEO to inhibit surface fungi did not affect the sausage drying process, pH, water activity, or color changes during ripening. These parameters change in a typical pattern for fermented dry-cured sausages during ripening. At the end of ripening, OEO-treated sausages had lower hardness and greater chewiness than the control but showed similar textural properties to sausages treated with potassium sorbate.