These are actually two slightly different trackwork standards, both with the aim of achieving more prototypical running than Finescale, they have become increasingly popular in recent years, and are really a variation on Finescale trackwork. There is no difference in vehicle wheel standard from the Guild Finescale Standard. The only difference being that trackwork is produced to a gauge of 31.5mm (0-MF) or 31.2mm (0-SF) whilst retaining the Finescale check rail/wing rail dimensions. The main reason for the change is to give a smoother ‘ride’ through pointwork for locomotives and rolling stock, particularly vehicles with small wheels. Many layouts built to these standards actually use plain trackwork to 32mm gauge, but have pointwork built with the reduced gauge over the point nose area (i.e. crossing).
There was an interesting discussion on the Forum about the benefits and otherwise of the 31.5mm track gauge, which can be read at this link: Forum discussion about O-MF.