The upload of the training data on Strava runs via the Rouvy sport platform https://rouvy.com/en/. Therefore you have to create a user account at both Strava and Rouvy.
After finishing a training in the o-synce mobile app, it is uploaded to the Rouvy sport platform. Please select the option 'Cloud' in 'Settings'.
At Strava you have the possibility to choose an automatic or a manual upload of the data.
The training summary will then appear in your Strava account.