Knowledge about the specific characteristics of floral transition process in oregano (Origanum vulgare L.) is scarce. Photoperiod is one of the environmental factors regulating oregano development. The aim of this study was to evaluate anatomical changes associated with the phenological transition from vegetative to floral stages of O. vulgare L. subsp. vulgare (Compacto ecotype) and O. vulgare L. subsp. hirtum (Link) Ietsw. (Criollo ecotype), in response to day length variations. These two traditional subspecies were grown under natural and artificially 6-h extended photoperiod. Phenology and internal floral transition did not follow a single evolution pattern, since these processes respond to complex gene-environment interactions. In the oregano subspecies studied, transition from vegetative to reproductive stages presented an intermediate step, consisting of the formation of an inflorescence meristem. Thus, floral transition occurred gradually and acropetally along the floral axis. Within the floral meristem, the appearance of reproductive perianthic pieces also occurred acropetally. Extended photoperiod seemed to act as a signal accelerating the floral transition process of the meristems, which confirms that oregano is a long-day plant. In addition, phenological and anatomical analyses confirmed that Criollo ecotype was most sensitive to photoperiodic lengthening, as evidenced by an earlier bud development all along the floral branch.