Assuming the joists are SPF select structural the floor will handle over 40 PSI live load and 10 PSI dead load (allowed up to 19'-6"). If the lumber rating is No 2 the load rating drops to something over 30 / 10. Assume it is NO. 2 unless you see stamps that it is SEL STR. The floor load rating is adequate for sleeping rooms but is marginal for an office. Especially if you will add a heavy desks, equipment and file cabinets. If you are looking at light duty low-occupancy, it may be adequate. Blocking and a better sub-floor will help stabilize things but you will either need a center beam or sistered joists to meet the higher load requirement. Rather than sister the joists you could just add between each joist. That would give you 8" O.C. joists which is overkill (over 60 /20). 3/4 to 1" T&G plywood nailed at 6" on edges and 12" field can stiffen the floor quite a bit compared to your current OSB sheets.
With the improved subfloor, you won't fall through, but you still should get an engineer's approval based on your intended use. Since you are creating occupied space, you are required to obtain a permit. You will be required to provide an engineer's certifcate of confomity and load calculations. So Steve's advice was correct. You can get a recommendation from an engineer that will meet your needs and comply with load requirements for a relatively cost. I also recommend you pursue that course.