by In the list below you can find some of the default accounts that you can use when conducting an assessment in Oracle databases.These passwords are the one that you can find more often.
SYSTEM:MANAGER
SCOTT:TIGER
SYS:CHANGE_ON_INSTALL
OUTLN:OUTLN
DBSNMP:DBSNMP
MDSYS:MDSYS
CTXSYS : CTXSYS
ORDSYS : ORDSYS
WKSYS : WKSYS
ANONYMOUS:ANONYMOUS
Penetration Testing Lab 
And these:
CTXSYS : CTXSYS – often DBA on Windows Oracle
ORDSYS : ORDSYS
WKSYS : WKSYS
Thank you Bryan for the contribution.I have updated the list with your accounts.
You’re welcome netbiosX. Keep up the good writing.
I’ve also found that the DBSNMP account often has read access to the majority of databases on the system. You might not be able to make changes, but if you can read the information that’s often good enough during a test.
@netbiosX – I have added some more default Oracle accounts here:
DIP: dip
DMSYS: DMSYS
DSSYS: DSSYS
EXFSYS:
ORACLE_OCM: ORACLE_OCM
ORDPLUGINS: ORDPLUGINS
ORDSYS: ORDSYS
PERFSTAT: PERFSTAT
SI_INFORMTN_SCHEMA: SI_INFORMTN_SCHEMA
TSMSYS: TSMSYS
WMSYS: WMSYS
XDB: XDB