Hi, if you want to get on the common route, stay away from all of the romanceable characters (don't help them, talk to them, don't get illustrations or other scenes where you get closer to them, etc. - ) and choose these in the first three chapters:
Chapter 1:
- Retreat
Chapter 2:
- Thanks.
- Follow them
Chapter 3:
- ... distressing.
- Go find someone. (The last one is the most important. It counts a lot.)
At the end of Chapter 4, choose to help in training sessions instead of looking after the troublesome trio. Osamu route is hard to get, so chapter 5 will most probably lead to the common route.