Objective: To adapt and partly validate a Danish online version of the patient-reported outcome measure (PROM) Oxford Physical Activity Questionnaire (“OPAQ”) and evaluate mobile phones and tablets as data capturing tool to identify potential problems and deficiencies in the PROM prior to implementation in the full study. Methods: The OPAQ was translated into Danish by a formalised forward-backward translation procedure. Face validity was examined by interviewing 12 school students aged 10–15, recruited from two Danish public schools. After modifications, the online version of the Danish OPAQ was pilot tested in a convenience sample of seven school students for 1 week. Simultaneous objective accelerometer data were captured during the registration period. Results: No major challenges were identified when translating OPAQ. Based on the interviews, the Danish version of OPAQ was perceived to be easy to understand in general, and the questions were relevant for tracking activities during the week. Five of the 12 participants had difficulties with understanding the introductory question: “what is your cultural background” in the original OPAQ. The interviews revealed that the participants recalling 7 days forgot to record some of the physical activity they had done during the week, indicating issues with the weekly recall method. After transforming to the online version, this was reported to be easy and quick to fill in (taking 1–3 min per day), and participants reported the daily design was helpful to remember activities. There was good correspondence between the online version and objective actigraphs with a tendency to underreport. Six participants reported 10–60 min less moderate to vigorous physical activity compared to the actigraphs, while one participant reported 3 min more. Conclusion: Participants found the online OPAQ quick and easy to complete during a 1-week period. Completing daily rather than weekly may help limit issues with recall. Overall, there was good agreement between the objective actigraphs and the OPAQ, though the OPAQ tended to slightly underreport moderate to vigorous physical activity. The Danish online version of OPAQ may be useful for capturing school students’ physical activity when objective measures are not feasible.