Background The importance of mental health promotion is irrevocable and is especially important at a young age. More mental health-promoting mobile apps have been developed in the last few years. However, their usability and quality have been rarely assessed. Objective The aim of this study is to investigate how adolescents assess the usability, quality, and potential goal achievement of Opp and NettOpp. Opp is a universal mental health–promoting mobile app aimed at 13- to 19-year-olds, and NettOpp is a mobile app for children and adolescents between 11 to 16 years of age that have experienced negative incidents online. Methods A total of 45 adolescents tested either Opp (n=30) or NettOpp (n=15) for a period of 3 weeks and answered a questionnaire. The System Usability Scale (SUS) was used to measure the usability of the apps. A SUS score above 70 indicates acceptable usability. Items from the Mobile Application Rating Scale were adapted for study purposes and used to measure the quality and perceived goal achievement that Opp and NettOpp might have on adolescents’ knowledge, attitudes, and intention to change behavior. Furthermore, adolescents could answer an open comment question. Results Opp had a mean SUS score of 80.37 (SD 9.27), and NettOpp’s mean SUS score was 80.33 (SD 10.30). In the overall evaluation, Opp and NettOpp were given a mean score of 3.78 (SD 0.42) and 4.20 (SD 0.56), respectively, on a 5-point scale, where 5 was best. Most adolescents who evaluated Opp rated that the app would increase knowledge about mental health and help young people deal with stress and difficult emotions or situations. Most adolescents who evaluated NettOpp agreed that the app would increase awareness and knowledge about cyberbullying, change attitudes toward cyberbullying, and motivate them to address cyberbullying. Some adolescents stated that Opp was difficult to navigate and consisted of too much text. Some of the adolescents that tested NettOpp stated that the app had crashed and that the design was a bit childish. Conclusions All in all, this study indicates that Opp and NettOpp have good usability and that adolescents are satisfied with both apps. It also indicates that the potential goal achievement of the apps, for example, increasing knowledge about mental health (Opp) or cyberbullying (NettOpp) is promising. While there are some comments from the users that are more difficult to solve (eg, Opp is too text-based), some comments helped improve the apps (eg, that the app crashed). Overall, the user evaluation provided valuable knowledge about how adolescents assess Opp and NettOpp. However, more extensive effectiveness studies are necessary to measure their actual goal achievement.

Background In times of increasing mental health problems among young people, strengthening efforts to improve mental health through mental health promotion and prevention becomes increasingly important. Effective measures that support young people in coping with negative thoughts, feelings, and stress are essential, not just for the individual but also for society. Objective The aim of this paper is to provide a description of a cluster randomized controlled trial that will be conducted to examine the effectiveness of Opp, a universal mental health–promoting mobile app for adolescents aged 13 to 19 years that provides information and exercises to better cope with stress, negative thoughts, and negative feelings. The protocol was developed in accordance with the SPIRIT checklist. Methods An effectiveness study will be conducted with 3 measurement points: preintervention (T1), 2 weeks after the intervention (T2), and about 1 month after the intervention (T3). Adolescents will be recruited from middle and high schools in Norway and randomly assigned to the intervention or control groups. Randomization will be conducted on the school level. Opp can be downloaded from the Google Play or App Store but is password protected with a 4-digit code, which will be removed after study completion. Participants in the intervention group will receive a text message with the code to unlock the app. The participants in the intervention group can use Opp without limits on length or time of use. Objective data on how long or how often the participants use the app will not be collected. However, the second and third questionnaires for the intervention group contain app-specific questions on, for example, the use of the app. Results Recruitment and data collection started in August and September 2022. So far, 381 adolescents have answered the first questionnaire. Data collection was expected to end in December 2022 but has had to be prolonged to approximately June 2023. The results of the study will be available in 2023 at the earliest. Conclusions This project will contribute unique knowledge to the field, as there are few studies that have examined the effects of universal health-promoting mobile apps for adolescents. However, several limitations have to be taken into account when interpreting the results, such as randomization on the school level, the short time frame in which the study was conducted, and the lack of objective data to monitor the use of the app. Trial Registration ClinicalTrials.gov NCT05211713; https://www.clinicaltrials.gov/ct2/show/NCT05211713 International Registered Report Identifier (IRRID) PRR1-10.2196/42119