With Odoo, there are so many work arounds.

You can create additional journal for beg. balance of sales and vendors. It should be done by vendor and customers to make reconcilliation of it's partner account

You may want to watch this video: https://odoobook.readthedocs.io/en/latest/accounting/import_opening.html

For managing opening balance for customer / supplier try following:

Go to Accounting -> Journal Entries -> Journal Entries and try to create new journal entry.

While creating the entry you need to select the journal under which you need the accounting entry to be done, which is mostly the 'Opening Journal'

Enter appropriate date (the starting date of new fiscal year).

Then you have to enter the details for customer/suppliers and their credit or debit respectively.

Once you are done with this post this journal entry details, which will generate the opening entries for customers / suppliers.