well I dont know how you can show AM and PM in Oracle

if you set NLS_LANG in your client then the client will use the default date format for the NLS_LANG you set

Let´s say you set
NLS_LANG=SPANISH_SPAIN.WE8ISO8859P1
in you winnt registry or enviroment variable in unix (depend where is the client located)

instead of receiving

DD/MM/YY HH24:MI:SS

as date format

you will get

DD/MM/YY

if you use AMERICAN_AMERICA:WE8ISO8859P1

you get

DD-MON-YY

If you dont set any

you get

DD/MM/YY HH24:MI:SS

Let´s see it this way, the parameters you set in client predominates the one from the server, the client side parameters has precedence (I think this is in English not sure the correct word )