Wow, heard back from HR the next day after stage 1. (I also did not expect the SDT questions but found that one the easiest in the end - I just did not manage to answer one of the questions where the numbers were not so nice to work with and I didn't have enough time to finish the proper calculation.)
Now the dreaded OPC. Scares me more than anything else in the selection process. Good luck to all fellow applicants.