You should be able to run a custom report (use Client Inventory). You'll have the option to choose what OS to report on or you can run it against all. There is a last SSID options as well. This will get you OS X devices and what SSID they were on.
If you select "include details about every client" you'll get get some additional information/fields to choose from in column format.....including SSID. Also, if you selecty "Limit to active devices = Active during report timeframe" you can specify your report start and end times.
You could also try a Client Session report which allows you to specify SSID and OS to run against.