Is it possible to query the history of previously locked/unlocked Oracle database user accounts? I've tried dba_user but there is no option to get history.
Going forward, you could implement auditing of
alter user commands which would capture when a user ID is unlocked, but would not capture when it got locked due to number of unsuccessful login attempts. Take a look at http://docs.oracle.com/database/121/DBSEG/audit_config.htm#DBSEG1115 for details. If you really need to know when it was locked, etc. as Balazs suggests, you could invoke SQL every hour/day/week that would insert into a table a copy of the columns of interest from dba_users:
create table my_log (as_of date, username varchar2(128), account_status varchar2(32), lock_date date);
Then every so often, do this:
insert into my_log (select sysdate, a.username, a.account_status, a.lock_date from dba_users a);