Dave,
The main advantage of the OS over the Fox is cooling fin area. The Fox is very light and Fox did that by keeping the case as small as possible which reduced the fin area. The OS can take more load (i.e. bigger prop, wider blades) than the Fox giving more pulling power. But it weights more so there is a trade-off. Generally, the Fox performs best below 42oz ish and the OS can get to maybe 45 - 47oz. There have been lots of heaver ships flow with both but if there is any significant wind they struggle at the higher weights.
Best, DennisT