The 0t tax code is applied when HMRC or your employer doesnot have much information about your tax information. They think no personal allowance, that means you get taxed on your whole income. It is common when one get a new job or your employer doesnot have your P45 or income details. This code is also used for second jobs or pension withdrawals. If you are on OT and think it is wrong then immediately contact HMRC.
