Objectives: This study aims to assess the usability of online educational applications (OEAs) among teachers during the unforeseen implementation of online distance learning. Specific objectives include characterizing respondents' profiles, evaluating the usability of each OEA, exploring the relationship between resources, technological skills, and OEA usability, and capturing teachers' lived experiences with OEAs in the context of promoting quality and sustainable education. Theoretical Framework: The study is anchored in the theory of connectivism, which emphasizes the importance of networks and the use of diverse online resources in learning, particularly in unexpected online learning scenarios, and its implications for achieving quality and sustainable education. Method: A convergent mixed-method design was utilized, combining quantitative data from a modified System Usability Scale and a researcher-made questionnaire with qualitative data from interviews. The study involved 105 teachers from partner schools who participated in training on various OEAs, including Kahoot, Quizizz, Padlet, and Mentimeter. Results and Discussion: The findings revealed that the usability of the OEAs was rated as marginally acceptable, with all applications scoring below the acceptable SUS average of 68. A significant relationship was found between technological skills, availability of resources, and OEA usability. Interviews highlighted that while teachers appreciated the training provided, the success of online education, and its alignment with quality and sustainable education goals, depends on adequate resources and ongoing support. Research Implications: The study underscores the need for continuous professional development and resource allocation to improve the usability of OEAs in sustaining quality online education. It suggests that schools should invest in both technological resources and training to enhance the effectiveness of online learning, thereby contributing to the broader objectives of quality and sustainable education. Originality/Value: This research provides valuable insights into the usability challenges of OEAs in emergency online learning contexts, contributing to the ongoing discourse on sustainable education practices and the pursuit of quality education during unforeseen disruptions.