OK, my bad... OSL is 'Object Source Lighting'. In a nutshell, it is a
secondary light source in addition to the primary light source. Lanterns, candles, lava, or a wizard's glowing orb for example. The usual way to paint the angel would be a primary light and direction - above, left or right, and perhaps the front. The candle is a secondary source and the yellow light would alter the colors across the knees, hands, face, and the front and sides of the hair, and perhaps parts of the sleeves as well. The light would be strongest at the hands and face and diffuse on areas that are further away. How much is up to the painter and their particular style. Normally, the piece is painted first, then the yellow light is applied as a glaze. There are instances where OSL would be the only light source such as a person holding a candle in an otherwise dark hallway. Most of the figure and hall would be painted in dark muted colors. The areas hit directly with the lantern light would be painted close to normal colors, then glazed with the yellow lantern light. Again, a nutshell...
English (and English English at that) tutorial video here:
https://www.youtube.com/watch?v=OKf6nvnsqBIGlen
