To provide a scalable solution to mental health problems caused by stress, we developed Oiva, a mobile phone intervention for improving mental and physical wellbeing. Oiva is based on Acceptance and Commitment Therapy (ACT) and its aim is to teach the users skills to increase their psychological flexibility. The application contains 45 text, audio, and video exercises. Two user studies demonstrated the usability and acceptability of the application and concept. The evaluations also revealed that the users expected guidance on the application for performing the intervention program. They also wanted to have possibilities to individualize the application by saving their own reflections about the exercises as notes. The preliminary evaluation results indicate that Oiva is a good starting point for the further design and research of mobile applications for reducing stress and improving wellness.