Use which schema to query 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;