The Oregon coast does not have a bad season, but it does have a wet one. July and August are the driest months and the most crowded, with coastal fog (the "marine layer") typically burning off by midday. The weekends fill accommodation quickly in summer; book ahead.
Spring (April to June) and autumn (September to October) offer the best balance: fewer crowds, relatively mild temperatures, and the dramatic wave action that makes the basalt coast here so spectacular. The grey whale migration passes close to shore in March–April (northward) and December–January (southward), making those months particularly rewarding for whale watching from the headlands.
Winter (November to February) is stormy, wet, and cold, but the coast at Yachats earns its reputation in bad weather. The wave energy on the basalt at Cape Perpetua during a winter storm is extraordinary. Many local visitors specifically come for winter storm watching — the fireplaces in the motels along the beachfront are not decorative. Accommodation is cheaper and the town is quiet.