This paper proposes a design guideline for selecting the pole and slot combination of an outer-rotor permanent magnet synchronous motor (PMSM) using a maximum slot occupation (MSO) coil. Because the MSO coil has a large conductor area, the AC Joule loss in the conductors may be increased at high frequencies. To ensure high-efficiency for the PMSM, it is necessary to reduce the loss. Thus, it is important to select the pole- and slot- combination that has the minimum AC Joule loss. The loss is caused by skin/proximity effects and variations in the slot leakage flux. The skin effect is due to the armature winding and the variation in the slot leakage flux is due to the field flux. A method for separating the AC Joule loss due to each component using the frozen permeability method is proposed. Based on the proposed method, the effect of each cause on the loss at various pole- and slot- combinations is analyzed in this study.