This step is identical to a standard 3x3: find a corner piece with the top color, bring it above its slot, and insert using R U R' U' . However, note that the Fisher Cube has two types of corners. Ensure you match the correct corner to its correct slot by checking the side colors, not just the shape.
Use your favorite 3x3 OLL algorithms. However, due to the 45-degree cut, you may see patterns that seem impossible. The PDF 136 dedicates 20 pages (pages 45–65) to Fisher-specific OLL cases. fisher cube solution pdf 136
Perform: (R U R' U') (R' F R F') and then resolve the last layer. This fixes the edge orientation without breaking middle layer. This step is identical to a standard 3x3: