First, test the connection between the transceiver and the OS2 by using the OS2's built-in local echo feature.
http://manuals.sharkrf.com/openspot2/en/control-calls.html
Note that only about 1 or 2 seconds can be played back on D-STAR. If this works correctly, then test the echo feature of a server which supports echo like REF001 module E. Make sure the URCALL is set to CQCQCQ in this case on your transceiver. Note that sometime the echo feature does not work on this server.
Also make sure that the TX power is set to 100% on the OS's Modem page. You can also try resetting your Modem settings by clicking on the button at the bottom of the Modem page.