Code:
SQL> connect a/a
Connected.
SQL> create or replace procedure foo is
  2  begin
  3   dbms_output.put_line('bar');
  4  end;
  5  /

Procedure created.

SQL> set serveroutput on
SQL> exec foo
bar

PL/SQL procedure successfully completed.

SQL> grant execute on foo to b;

Grant succeeded.

SQL> connect b/b
Connected.
SQL> set serveroutput on
SQL> exec a.foo
bar

PL/SQL procedure successfully completed.