There is a bus that goes there a couple of times a day in Summer. You will, as always, find the schedules on www.sbb.ch
From Grimsel Hospiz there there is a cable car up to the Oberaarsee, and from there you can then start your hike.
A lot of info is on this website:
https://www.grimselwelt.ch/
The whole Grimsel Area is a nice destination if you want something a bit more off the beaten path, not overtouristed. With your BO pass the bus is included, but for the cable cars around Hospiz you will need to pay a reduced fare.
The bus is not very frequent though, so you may have to plan around it.
If I was doing this hike (and I might well do this this summer), I would do it like this:
- Take the earliest bus of the day to the Grimsel Pass Summit (arr. 8:48), and start the hike there. Hike to Sidelhorn, and then down to Oberaarsee. You can then add the trek out to the Glacier and back if you want (adds another 2 hours).
- Take cable car back to Grimsel Hospiz, lax and take the bus back from there.
The hike will take you about three hours (or five if you go all the way to the Glacier), and it is a proper mountain hike, so unless you have done this before, and know how to dress and prepare for that you maybe shouldn't do this. It does pass through some difficult terrain, and follows a ridge for some part of the route. You will need a head for heights for this.
An easier alternative would be to take the bus to Grimsel Hospiz, take the cable car to the Oberaarsee, and then just hike from there to the Glacier and back.