If you don’t specify NLS_LANG in your shell when starting OAS, OAS goes and sets it for you.
Fix is to specify NLS_LANG in any OAS startup scripts you use, or edit these files:
$ grep -i NLS_LANG $ORACLE_HOME/Apache/Apache/bin/apachectl
NLS_LANG=${NLS_LANG="ENGLISH_UNITED KINGDOM.WE8ISO8859P1"}; export NLS_LANG
$ grep -i NLS_LANG $ORACLE_HOME/opmn/bin/opmnctl
NLS_LANG=${NLS_LANG="ENGLISH_UNITED KINGDOM.WE8ISO8859P1"}; export NLS_LANG
$
Reference 10gR2 Oracle Application Server Globalization Support Guide, Chapter 5.