The optimum working frequency provides the best long-range HF communication. Compared with the maximum usable frequency (MUF), it is usually:

The Maximum Usable Frequency (MUF) is the absolute highest frequency that will support skywave propagation between two specific points at a given time. However, operating exactly at the MUF can be unreliable because the ionosphere is constantly shifting; even minor fluctuations can cause the MUF to momentarily drop, leading to sudden signal loss. To ensure reliable communication, operators use the Optimum Working Frequency (OWF or FOT), which is typically chosen to be about 15% to 20% below the MUF. By operating slightly lower than the MUF, you establish a "reliability buffer" that prevents sudden signal dropouts while still benefiting from the low D-layer absorption that higher frequencies enjoy.