with data as
 (select 'August 2015' v
    from dual
  union all
  select '2014 may' v
    from dual
  union all
  select 'June 2013' v from dual
  
  )
select * from data

SCOTT@orcl> create or replace function fix_date
  2    (p_string	 in varchar2)
  3    return		    date
  4  as
  5    v_date		    date;
  6    e_invalid_month	exception;
  7    pragma exception_init (e_invalid_month, -01843);
  8  begin
  9    begin
 10  	 v_date := to_date (lower (p_string), 'month yyyy', 'NLS_DATE_LANGUAGE = American');
 11    exception
 12  	 when e_invalid_month then
 13  	   begin
 14  	     v_date := to_date (lower (p_string), 'yyyy mon', 'NLS_DATE_LANGUAGE = American');
 15  	   exception
 16  	     when others then
 17  	     v_date := to_date (null);
 18  	   end;
 19  	 when others then
 20  	   v_date := to_date (null);
 21    end;
 22    return v_date;
 23  end fix_date;
 24  /

Function created.

SCOTT@orcl> show errors
No errors.
SCOTT@orcl> with data as
  2   (select 'August 2015' v
  3  	 from dual
  4    union all
  5    select '2014 may' v
  6  	 from dual
  7    union all
  8    select 'June 2013' v from dual
  9    )
 10  select to_char (fix_date (v), 'dy dd-mon-yyyy') fixed_date from data
 11  /

FIXED_DATE
---------------------------------
sat 01-aug-2015
thu 01-may-2014
sat 01-jun-2013

3 rows selected.

with data as
 (select 'August 2015' v
    from dual
  union all
  select '2014 may' v
    from dual
  union all
  select 'June 2013' v from dual
)
select to_date(regexp_substr(v,'[[:alpha:]]+') || regexp_substr(v,'\d+'),'monthyyyy') dt
  from data
/

DT
---------
01-AUG-15
01-MAY-14
01-JUN-13

SQL>