Oversize Guide
If the size of the hole is much greater than the fish size, there is a possibility that a standard guide may pass the top of the fish without guiding it to the centre of the overshot. In that case running an oversize guide has greater chances of success in catching the fish.
The guide is run below the overshot assembly. With string rotation, it scoops the fish to the centre of the hole and helps the grapple in overshot swallow the top to engage and retrieve it.