As the other answers state, changing the user's profile (e.g. the 'DEFAULT' profile) appropriately will lead to passwords, that once set, will never expire.
However, as one commenter points out, passwords set under the profile's old values may already be expired, and (if after the profile's specified grace period) the account locked.
The solution for expired passwords with locked accounts (as provided in an answering comment) is to use one version of the ALTER USER command:
ALTER USER xyz_user ACCOUNT UNLOCK;
However the unlock command only works for accounts where the account is actually locked, but not for those accounts that are in the grace period, i.e. where the password is expired but the account is not yet locked. For these accounts the password must be reset with another version of the ALTER USER command:
ALTER USER xyz_user IDENTIFIED BY new_password;
Below is a little SQL*Plus script that a privileged user (e.g. user 'SYS') can use to reset a user's password to the current existing hashed value stored in the database.
EDIT: Older versions of Oracle store the password or password-hash in the pword column, newer versions of Oracle store the password-hash in the spare4 column. Script below changed to collect the pword and spare4 columns, but to use the spare4 column to reset the user's account; modify as needed.
REM Tell SQL*Plus to show before and after versions of variable substitutions. SET VERIFY ON SHOW VERIFY REM Tell SQL*Plus to use the ampersand '&' to indicate variables in substitution/expansion. SET DEFINE '&' SHOW DEFINE REM Specify in a SQL*Plus variable the account to 'reset'. REM Note that user names are case sensitive in recent versions of Oracle. REM DEFINE USER_NAME = 'xyz_user' REM Show the status of the account before reset. SELECT ACCOUNT_STATUS, TO_CHAR(LOCK_DATE, 'YYYY-MM-DD HH24:MI:SS') AS LOCK_DATE, TO_CHAR(EXPIRY_DATE, 'YYYY-MM-DD HH24:MI:SS') AS EXPIRY_DATE FROM DBA_USERS WHERE USERNAME = '&USER_NAME'; REM Create SQL*Plus variable to hold the existing values of the password and spare4 columns. DEFINE OLD_SPARE4 = "" DEFINE OLD_PASSWORD = "" REM Tell SQL*Plus where to store the values to be selected with SQL. REM Note that the password hash value is stored in spare4 column in recent versions of Oracle, REM and in the password column in older versions of Oracle. COLUMN SPARE4HASH NEW_VALUE OLD_SPARE4 COLUMN PWORDHASH NEW_VALUE OLD_PASSWORD REM Select the old spare4 and password columns as delimited strings SELECT '''' || SPARE4 || '''' AS SPARE4HASH, '''' || PASSWORD || '''' AS PWORDHASH FROM SYS.USER$ WHERE NAME = '&USER_NAME'; REM Show the contents of the SQL*Plus variables DEFINE OLD_SPARE4 DEFINE OLD_PASSWORD REM Reset the password - Older versions of Oracle (e.g. Oracle 10g and older) REM ALTER USER &USER_NAME IDENTIFIED BY VALUES &OLD_PASSWORD; REM Reset the password - Newer versions of Oracle (e.g. Oracle 11g and newer) ALTER USER &USER_NAME IDENTIFIED BY VALUES &OLD_SPARE4; REM Show the status of the account after reset SELECT ACCOUNT_STATUS, TO_CHAR(LOCK_DATE, 'YYYY-MM-DD HH24:MI:SS') AS LOCK_DATE, TO_CHAR(EXPIRY_DATE, 'YYYY-MM-DD HH24:MI:SS') AS EXPIRY_DATE FROM DBA_USERS WHERE USERNAME = '&USER_NAME';