SCOTT@orcl_11gR2> create table test
  2    (id	  number,
  3  	lastname  varchar2(30))
  4  /

Table created.

SCOTT@orcl_11gR2> insert into test( id, lastname )
  2  values ( 1, 'Muller' )
  3  /

1 row created.

SCOTT@orcl_11gR2> insert into test( id, lastname )
  2  values ( 2, 'Müller' )
  3  /

1 row created.

SCOTT@orcl_11gR2> insert into test( id, lastname )
  2  values ( 3, 'other' )
  3  /

1 row created.

SCOTT@orcl_11gR2> commit
  2  /

Commit complete.

SCOTT@orcl_11gR2> select * from test
  2  /

        ID LASTNAME
---------- ------------------------------
         1 Muller
         2 Müller
         3 other

3 rows selected.

SCOTT@orcl_11gR2> begin
  2    ctx_ddl.create_preference ('test_lex', 'basic_lexer');
  3    ctx_ddl.set_attribute ('test_lex', 'base_letter', 'yes');
  4    ctx_ddl.set_attribute ('test_lex', 'base_letter_type', 'generic');
  5  end;
  6  /

PL/SQL procedure successfully completed.

SCOTT@orcl_11gR2> create index test_idx
  2  on test (lastname)
  3  indextype is ctxsys.context
  4  parameters ('lexer  test_lex')
  5  /

Index created.

SCOTT@orcl_11gR2> select token_text from dr$test_idx$i
  2  /

TOKEN_TEXT
----------------------------------------------------------------
MULLER
OTHER

2 rows selected.

SCOTT@orcl_11gR2> select * from test where contains (lastname, 'Muller') > 0
  2  /

        ID LASTNAME
---------- ------------------------------
         1 Muller
         2 Müller

2 rows selected.

SCOTT@orcl_11gR2> select * from test where contains (lastname, 'Müller') > 0
  2  /

        ID LASTNAME
---------- ------------------------------
         1 Muller
         2 Müller

2 rows selected.