Oshitazu (preliminary sketch) is an outline of the work sketched in full size before starting on the actual piece. It is also referred to as oshitae (rough sketch) or soko (draft).

In Japanese paintings, oshitazu is drawn on a separate medium to the final work, transferring ideas worked over on sketches and koshitazu (small sketches), in order to check the composition and arrangement of the finer details. This is done because materials used in Japanese paintings are difficult to amend. Preparing oshitazu helps the painter to plan out procedures for producing the painting and to predict problems. It is distinct from ordinary sketches and koshitazu because appearances and impressions change with variations in scale and the shapes can be rendered unsuitable if simply enlarged by photocopier. It is thus a medium for making such adjustments and modifications. Sketches can be traced on tracing paper or a thin piece of washi paper and be moved around on the screen to determine the composition and layout. Oshitazu should be amended as many times as required to achieve the desired image before settling on the final composition.

There are several ways of transferring koshitazu onto an oshitazu. A common method is to draw an equally spaced grid of vertical and horizontal lines over the koshitazu, draw a grid with the same number of squares on the oshitazu, and then draw in the details in each corresponding square. Another method would be to produce an enlarged copy of the koshitazu with a photocopier and use that.

Depending on the intended expression, an artist may decide to produce the final work without preparing an oshitazu. For beginners, however, it is important to follow the procedures step by step also to gain an understanding of the significance of the basic procedures for producing Japanese paintings.