@mdkdumul,
You can add opening balance by navigating to: “Dashboard -> Accounting -> Journal Entry -> Add new journal entry”.
One other thing: “Opening balance – 0”
It’s showing “0” due to pagination. When there will be so many transactions in this account and there will be several pages in this section then if you go to the second or third page, you will see the opening balance as the total of the previous page.
Hope this was understandable and helpful.
Thank you 🙂