OUTER JOIN is documented in PostgreSQL.
This is also documented in Oracle SQL Reference which says:
Oracle recommends that you use the FROM clause OUTER JOIN syntax
  rather than the Oracle join operator. Outer join queries that use the
  Oracle join operator (+) are subject to the following rules and
  restrictions, which do not apply to the FROM clause OUTER JOIN syntax:
You cannot specify the (+) operator in a query block that also contains FROM clause join syntax.
The (+) operator can appear only in the WHERE clause or, in the context of left-correlation (when specifying the TABLE clause) in the
  FROM clause, and can be applied only to a column of a table or view.
If A and B are joined by multiple join conditions, then you must use the (+) operator in all of these conditions. If you do not, then
  Oracle Database will return only the rows resulting from a simple
  join, but without a warning or error to advise you that you do not
  have the results of an outer join.
The (+) operator does not produce an outer join if you specify one table in the outer query and the other table in an inner query.
NB: in Oracle PL/SQL there is neither += nor +:= operator:
SQL> declare
  2  i numeric := 0;
  3  begin
  4   i+:=1;
  5  end;
  6  /
 i+:=1;
  *
ERROR at line 4:
ORA-06550: line 4, column 3:
PLS-00103: Encountered the symbol "+" when expecting one of the following:
:= . ( @ % ;
The symbol "+" was ignored.
SQL> --
SQL> declare
  2  i numeric := 0;
  3  begin
  4   i+=1;
  5  end;
  6  /
 i+=1;
  *
ERROR at line 4:
ORA-06550: line 4, column 3:
PLS-00103: Encountered the symbol "+" when expecting one of the following:
:= . ( @ % ;
SQL> show errors
No errors.
SQL> --
SQL> declare
  2  i numeric := 0;
  3  begin
  4   i := i + 1;
  5  end;
  6  /
PL/SQL procedure successfully completed.
SQL>