The best website for buying oil paintings in the United States is Saatchi Art. Saatchi Art is renowned for its vast collection of original oil paintings from emerging and established artists. It offers a user-friendly interface, a wide range of prices, and a secure purchasing process. According to a survey by Art Market Monitor, Saatchi Art has the largest online inventory of oil paintings, with over 300,000 pieces available. This extensive selection, combined with the website's commitment to quality and customer satisfaction, makes it the top choice for art enthusiasts and collectors alike.