SQL> create or replace procedure p_test(a in varchar2,b in varchar2,c out varchar2)
2 is
3 begin
4 c := a||b;
5 end p_test;
6 /

Procedure created

SQL> declare
2 v_sql varchar2(1000);
3 c varchar2(1000);
4 begin
5 v_sql:='begin p_test(:v1,:v2,:v3); end;';
6 execute immediate v_sql using in '1',in '2',out c;
7 dbms_output.put_line(c);
8 end;
9 /

12

PL/SQL procedure successfully completed

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Specifying Parameter Modes
With the USING clause, you need not specify a parameter mode for input bind arguments because the mode defaults to IN. With the RETURNING INTO clause, you cannot specify a parameter mode for output bind arguments because, by definition, the mode is OUT. An example follows:
DECLARE
sql_stmt VARCHAR2(200);
dept_id NUMBER(2) := 30;
old_loc VARCHAR2(13);
BEGIN
sql_stmt :=
'DELETE FROM dept WHERE deptno = :1 RETURNING loc INTO :2';
EXECUTE IMMEDIATE sql_stmt USING dept_id RETURNING INTO old_loc;
...
END;

When appropriate, you must specify the OUT or IN OUT mode for bind arguments passed as parameters. For example, suppose you want to call the following standalone procedure:
CREATE PROCEDURE create_dept (
deptno IN OUT NUMBER,
dname IN VARCHAR2,
loc IN VARCHAR2) AS
BEGIN
SELECT deptno_seq.NEXTVAL INTO deptno FROM dual;
INSERT INTO dept VALUES (deptno, dname, loc);
END;

To call the procedure from a dynamic PL/SQL block, you must specify the IN OUT mode for the bind argument associated with formal parameter deptno, as follows:
DECLARE
plsql_block VARCHAR2(500);
new_deptno NUMBER(2);
new_dname VARCHAR2(14) := 'ADVERTISING';
new_loc VARCHAR2(13) := 'NEW YORK';
BEGIN
plsql_block := 'BEGIN create_dept(:a, :b, :c); END;';
EXECUTE IMMEDIATE plsql_block
USING IN OUT new_deptno, new_dname, new_loc;
IF new_deptno > 90 THEN ...
END;