These are two standards for single mode fibre optic cabling from a total of five types of fibre that are generically used today known of “OF types” (OM1, OM2, OM3 for multimode and OS1,OS2 for single mode.)
OS1 is for indoor use i.e. Campus, Data Centre. Cabling is is tight buffered (i.e. manufactured into solid medium).
OS2 is for outdoor or loose tube use i.e. street, underground/burial, etc. (loose tube = not held in medium but blown or otherwise inserted into carrier).
Indoor fibre is more tolerant of bending (called B1.3). The fibre is more plastic and able to bend plus the buffered cable reduces the risk of catastrophic damage.
Outdoor fibres are bend sensitive and thus more likely to break during install unless care is taken. (not sure if this is due to the material of the fibre core or because the bending is an uncontrolled action in a loose tube medium).
OS1 fibre has greater loss per kilometre than OS2 outdoor fibre (as you would expect) and probably cost less because of this. Following table shows attenuation of 1.0db/km for OS1 compared to 0.4db/km for the outdoor cable (its cheaper to make cable at higher loss levels.
Cisco website for a FIA PDF file : https://learningnetwork.cisco.com/servlet/JiveServlet/previewBody/3791-102-1-10509/Understanding%20OM1,OM2,OM3,%20OS1,OS2%20Fiber.pdf