Beginners in oil painting often make common mistakes that can hinder progress. Overloading the brush with paint leads to muddy textures and difficulty blending. Skipping proper surface preparation, like priming the canvas, can affect paint adhesion. Beginners may also overlook investing in quality materials, leading to frustration with poor results. Lastly, rushing through a piece instead of focusing on foundational skills, like color mixing and composition, can hinder artistic development.