This study addresses the presence and location of natural pigmentation, potentially pathological changes in pigment, interruptions of the natural lining (scars), roughness, and erosions/contusion (bruising) in and around the corners of the lips of 206 horses presented to a veterinarian for routine preventative dental treatment. After sedation, photographs were taken and later evaluated for the presence of lesions. During the photographic analysis, the Oral Commissure Assessment (OCA) protocol was developed to map precisely the areas of skin and mucosa around the corners of the lips, and the presence of lesions was recorded for each area. Potentially pathological pigment changes occurred more frequently in horses with a higher level of training (p = 0.04) and in light-coloured horses (p = 0.0004), but there was no association with the current use of a bit or the discipline that the horse participated in (p = 0.20). Scars occurred more frequently in horses competing at a higher level. Only two horses had contusions or erosions, five had ulcers, and none showed bleeding; these numbers were too low for statistical analysis. Using the OCA protocol provides a detailed method for categorizing and recording lesions in and around the corners of the lips, including natural vs. potential and/or definite pathological character.