Oases are the highlights of arid and semi-arid ecosystems, supporting local livelihoods and regional development. Despite the extensive coverage of arid and semi-arid lands in East Asia, the spatiotemporal dynamics of oases in this region remain poorly understood, particularly under varying socioeconomic and policy conditions. In this study, we used Landsat imagery on the Google Earth Engine platform to generate 30-m resolution maps of oasis distribution in East Asia from 2000 to 2020, by employing the LandTrendr-PCA (principal component analysis) collaborative monitoring framework and various machine learning techniques. We systematically analyzed oasis evolution and identified the main influential factors by using a Random Forest model. Our results reveal that oases in East Asia, predominantly located in northwestern China and the eastern and western sides of southern Mongolia, expanded by 5.3 million hectares (Mha), or 12.8% from 2000 to 2020, equating to an average annual growth of 0.3 Mha, and over 90% of this expansion occurred in China. However, the study also revealed that approximately 9.5 Mha of oasis areas degraded into desert during the same period, about 67% of which occurred in Mongolia. Furthermore, our analysis highlights divergent influential factors of oasis dynamics in China and Mongolia, which went through different paths of socioeconomic development, especially in terms of water resource availability and climate change. In China, water resource availability was the primary factor that drove oasis dynamics, whereas in Mongolia climate change played a more prominent role. This study establishes a robust framework for long-term oasis monitoring, addresses critical gaps in oasis mapping, and offers valuable insights for sustainable oasis management in arid and semi-arid regions. Plain language summary: In the water-scarce regions of East Asia, oases are vital ecosystems that sustain agriculture and local livelihoods. Their dynamics and distribution, influenced by complex natural and social interactions, remain poorly understood. We generated distribution maps of East Asian oases from 2000 to 2020, thereby addressing this knowledge gap by examining their spatiotemporal evolution and identifying the main natural and socioeconomic factors that may affect these changes. Our findings show that oases in East Asia has expanded by 5.3 million hectares (Mha) or 12.8%, and over 90% of the oasis expansion occurred in northwest China. In the meantime, approximately 9.5 Mha of the oases in East Asia were transformed into deserts, and Mongolia accounted for about 67% of such land degradation. In China, the expansion of oases was largely related to water resources, whereas in Mongolia, climate change had a more pronounced impact. This research provides new insights to our understanding of oasis dynamics and supports sustainable management efforts in waterscarce environments.