Use which schema to query V$OSSTAT I need to query the values of V$OSSTAT and am able to do so using tools like dbVisualizer. But when I take the identical query and put it into some Javacode I fail to find the table.

I had this problem before with specific application schemas and was able to solve this by putting the applications schema name in front of the table simiar to this: SELECT NAME FROM FOOAPP.USERS.

But in this case I simply don't know which schema to use for gaining access to V$OSSTAT. I tried prefixing the view / table with SYS and SYSTEM already.

SELECT STAT_NAME, VALUE, COMMENTS FROM SYS.V$OSSTAT
SELECT STAT_NAME, VALUE, COMMENTS FROM SYSTEM.V$OSSTAT
There is no such object as SYS.V$OSSTAT, because V$OSSTAT is a public synonym.
Because of this, there is no need to use a schema specifier for it.
The actual view is called V_$OSSTAT (same for other V$ views as well).
If you want a user to be able to select from that view, grant the privilege on that:

grant select on sys.v_$osstat to user;