Short answer is RTFM, but from here.
“Note: Oracle 10g still keeps scott / tiger username and password (UID=scott, PWD=tiger) from the old version of oracle. In the old version of oracle, scott/tiger user ID is available by default, but not in oracle 10g. If you want to use scott /tiger account, you must unlock it by clicking “Password Management” at the last window.
Password Management window will appear like the one shown below. Find the user name “Scott” and uncheck the “Lock Account?” column for the user name. “