Hi, a combination of trains that is not suggested for the trip Thalwil to Olten is to take:
RE 3760 Thalwil 06:39 -> Zurich HB 06:48
IR 2362 Zürich HB 06:53 -> Olten 07:28
I know people who have been doing this combination for a year and they never had a problem catching the train to Olten.
Regards
Thanasis