There is no reason to wait to complete your AOW certification.
Your open water course teaches you the basic skills that you need to know to safely dive. Your AOW course builds on this and focuses on 5 specific types of diving. Two segments of your course will focus on the planning considerations for deep diving (deeper than 60 feet) and for navigating underwater. The other three modules are geared toward your specific interests and may encompass peak buoyancy, wreck diving, search&recovery, altitude diving, fish ID, underwater imaging, etc. Each will help you broaden your diving knowledge and make you more comfortable in the water. The advanced course focuses on the pre-dive briefing and de-brief as well as gaining new experiences in a controlled environment with an instructor (it is far less focused on conducting “skills” than your open water course).
Gaining experience through diving is also important to being comfortable in the water. The key difference is diving within your training and experience - the AOW course provides a controlled environment for you to expand your diving training and experience.