If you use the default password, is likely to cause some security risks to your database, you can use the following query to get those users use the default password
select username "User(s) with Default Password!"
from dba_users
where password in
('E066D214D5421CCC', -- dbsnmp
'24ABAB8B06281B4C', -- ctxsys
'72979A94BAD2AF80', - mdsys
'C252E8FA117AF049', -- odm
'A7A32CD03D3CE8D5', -- odm_mtr
'88A2B2C183431F00', -- ordplugins
'7EFA02EC7EA6B86F', -- ordsys
'4A3BA55E08595C81', -- outln
'F894844C34402B67', -- scott
'3F9FBD883D787341', -- wk_proxy
'79DF7A1BD138CF11', -- wk_sys
'7C9BA362F8314299', - wmsys
'88D8364765FCE6AF', -- xdb
'F9DA8977092B7B81', -- tracesvr
'9300C0977D7DC75E', -- oas_public
'A97282CE3D94E29E', - websys
'AC9700FD3F1410EB', -- lbacsys
'E7B5D92911C831E1', -- rman
'AC98877DE1297365', - perfstat
'66F4EF5650C20355', -- exfsys
'84B8CBCA4D477FA3', - si_informtn_schema
'D4C5016086B2DC6A', -- sys
'D4DF7931AB130E37') -- system
/