Usually, No Objection Certificate (NOC) is required to apply for a job or attend a job interview when you are already in employment. It is mandatory for Central Government, State Government and PSUs employees who want to apply for another job.
When you are in a job and want to apply for another job, you will have to apply through proper channel through present employer. Your bank will send your application with a forwarding note to the department, where you are applying. However, you cannot make a request for NOC during probation period. Applying through proper channel will keep your present employer in confidence and also you will be able to get your previous service and related benefits that will be carried forward to the new job.
As you have mentioned in your query that you had applied for the IBPS PO before joining the current bank job. This means you were not employed in bank at that time. And obviously, you had not mentioned it in your application form of IBPS PO also. Since, you applied for the other job (IBPS PO) before your present employment, there is no need to get a NOC now.
Now you should start preparation for interview and appear in interview without any worry in mind. After selection you will have to resign from your present job as per the terms and conditions and also serve notice period. And you will be able to join another job only after getting reliving letter from the current bank.