I want to be sure which Oracle user you are referring to - you DID say Admin user, but here are some clarifications:
In Oracle, there is one ''DB Admin'' user and multiple ''schema-users'', one for each Schema used by QC -- i.e. one for qcsiteadmin_db, one for each project schema.
The term ''owner'' does not really apply to the DB-Admin user, but the ''owner'' of each QC-related schema would be the ''schema-user''.
Generally, we do NOT change the ''schema-users'' passwords and they are usually all the same password.
Since you specifically mention the ''Db-Admin'' user -- See the DB-server tab in Site admin, it shows 2 users for Oracle and allows you to set their password.
The upper one is the DB-Admin (user and password), the lower one is the password (password only) to be used when creatiing NEW projects (and their schemas).
If you change the DB-Admin one, it will impact creating NEW projects (and schemas) and doing any project-level maintenance or upgrade/patching, but is not generally used during day-today usage of QC.
So, if you change the DB-Admin level user password in Oracle, you need to immediately change it in the DB-Servers tab in Site Admin, and test it while still in Site Admin -- there should be a ''test'' button when changing the password.