Ug. Storing dates in strings. This makes me sad :(

You need to use different masks depending on the date format.

Here's one way to do this with regular expressions:

create table t (
  x varchar2(30)
);
insert into t values ('1/1/2014 14:23');
insert into t values ('30/09/2014 12:31:11 PM'); 

with converted as (
select case
 when regexp_like(x, '(AM|PM)') then
   to_timestamp(x, 'dd/mm/yyyy hh:mi:ss am')
 else
   to_timestamp(x, 'dd/mm/yyyy hh24:mi')
 end ts
from   t
)
  select extract(year from ts) yr from converted;

        YR
----------
      2014
      2014