A set of realistic odd-one-out stimuli gathered "in the wild". Each image in the Odd-One-Out (O3) dataset depicts a scene with multiple objects similar to each other in appearance (distractors) and a singleton (target) distinct in one or more feature dimensions (e.g. color, shape, size). All images are resized so that the larger dimension is 1024px. Targets represent approx. 400 common object types such as flowers, sweets, chicken eggs, leaves, tiles and birds. Pixelwise masks are provided for targets and distractors. Annotations are generated using CVAT.