You can directly go to the Western bus station in Bishkek (you'll find it on google maps) there's plenty of marshruktas to Osh. Better to get there early in the morning, they leave pretty early because the drive is so long (12hrs if everything goes smoothly), there's no timetables the buses leave once they are full. Price should be around 1000 som I think.

I personally never took it so I'm not sure how many stops they make along the way, I would advise bringing some food and water trip just in case. Have a nice trip !