....
BEGIN
    v_string := p_string<font color=blue> || ';'</font>;
LOOP
....

v_string := p_string || ';';

SQL> CREATE OR REPLACE FUNCTION extract_next (
  2      p_string    IN  VARCHAR2
  3  ,   p_error_out OUT DBMS_SQL.VARCHAR2S
  4  ) 
  5  RETURN BOOLEAN IS
  6      v_string    VARCHAR2(256); 
  7      v_pos       NUMBER;
  8      v_errmsg    VARCHAR2(200);
  9      v_cnt       NUMBER := 0; 
 10  BEGIN
 11      v_string := p_string <font color=blue>|| ';'</font>;
 12      LOOP 
 13          v_pos := instr(v_string, ';'); 
 14          EXIT WHEN (nvl(v_pos, 0) = 0); 
 15          v_cnt := v_cnt + 1; 
 16          p_error_out(v_cnt) := (trim(substr(v_string, 1, v_pos - 1))); 
 17          v_string := substr(v_string, v_pos + 1); 
 18      END LOOP; 
 19      RETURN TRUE;
 20  EXCEPTION
 21      WHEN OTHERS THEN 
 22          v_errmsg := SUBSTR(SQLERRM, 1,200); 
 23          RETURN FALSE;
 24  END extract_next;
 25  /
  
Function created.
  
SQL> DECLARE
  2      b   BOOLEAN;
  3      x   DBMS_SQL.VARCHAR2S;
  4  BEGIN
  5      b := extract_next ('&p_string',x);
  6      IF (b) THEN
  7          DBMS_OUTPUT.PUT_LINE('b = TRUE');
  8      ELSE
  9          DBMS_OUTPUT.PUT_LINE('b = FALSE');
 10      END IF;
 11      FOR i IN 1..x.COUNT LOOP
 12          DBMS_OUTPUT.PUT_LINE('x(' || i || ') = ' || x(i));
 13      END LOOP;
 14  END;
 15  /
Enter value for p_string: OM-1001
old   5:     b := extract_next ('&p_string',x);
new   5:     b := extract_next ('OM-1001',x);
b = TRUE
x(1) = OM-1001
  
PL/SQL procedure successfully completed.
  
SQL> /
Enter value for p_string: OM-1001;TG-1990;SM-1001
old   5:     b := extract_next ('&p_string',x);
new   5:     b := extract_next ('OM-1001;TG-1990;SM-1001',x);
b = TRUE
x(1) = OM-1001
x(2) = TG-1990
x(3) = SM-1001
  
PL/SQL procedure successfully completed.
  
SQL>