As a paradigmatic microwave photonic resonant system that is capable of producing high-quality self-sustained microwave oscillations, the optoelectronic oscillator (OEO) has been intensively investigated in recent years, and a diversity of new insights and breakthroughs have been proposed and demonstrated. In this perspective, we discuss the recent progress, opportunities, and challenges of OEOs. Specifically, an overview of different OEO schemes for single-frequency and complex microwave signal generation is provided. Emerging advances in integrated OEO and applications of OEO are briefly reviewed. We also discuss the remaining challenges and opportunities in this field.