Code:
scott@oracle> create or replace function twice(a in number) return number
  2  as
  3  begin
  4    return (a*2);
  5  end;
  6  /

Function created.

scott@oracle> declare
  2  b number;
  3  begin
  4    execute immediate 'begin :b:=twice(7); end;' using out b;
  5    dbms_output.put_line(b);
  6  end;
  7  /
14

PL/SQL procedure successfully completed.
See http://download-west.oracle.com/docs...ynam.htm#13131