diff --git a/account_operating_unit/__init__.py b/account_operating_unit/__init__.py
index 1fc45468f5..3244c50a00 100644
--- a/account_operating_unit/__init__.py
+++ b/account_operating_unit/__init__.py
@@ -2,3 +2,4 @@
 
 from . import models
 from . import report
+from . import wizard
diff --git a/account_operating_unit/tests/__init__.py b/account_operating_unit/tests/__init__.py
index 68197dab28..4ee788f44a 100644
--- a/account_operating_unit/tests/__init__.py
+++ b/account_operating_unit/tests/__init__.py
@@ -5,4 +5,5 @@
 from . import test_cross_ou_journal_entry
 from . import test_operating_unit_security
 from . import test_payment_operating_unit
+from . import test_payment_register_journal_ou
 from . import test_account_reconcile
diff --git a/account_operating_unit/tests/test_account_operating_unit.py b/account_operating_unit/tests/test_account_operating_unit.py
index 2f46fcef34..506107c79c 100644
--- a/account_operating_unit/tests/test_account_operating_unit.py
+++ b/account_operating_unit/tests/test_account_operating_unit.py
@@ -2,6 +2,7 @@
 # © 2019 Serpent Consulting Services Pvt. Ltd.
 # License LGPL-3.0 or later (https://www.gnu.org/licenses/lgpl.html).
 
+from odoo import fields
 from odoo.models import Command
 from odoo.tests import tagged
 
@@ -181,3 +182,43 @@ def _prepare_invoice(self, operating_unit_id, name="Test Supplier Invoice"):
             "invoice_line_ids": lines,
         }
         return inv_vals
+
+    def test_payment_register_journals_filtered_by_ou(self):
+        """Payment register wizard should only show journals matching the
+        invoice's operating unit. A user with access to a single OU must
+        not see journals from other OUs, and creating a payment must not
+        raise AccessError."""
+        inv_vals = self._prepare_invoice(self.b2b.id, name="Test B2B Invoice")
+        inv_vals["invoice_date"] = fields.Date.today()
+        invoice = (
+            self.move_model.with_user(self.user1)
+            .with_context(default_move_type="in_invoice")
+            .create(inv_vals)
+        )
+        invoice.action_post()
+
+        ctx = {
+            "active_model": "account.move",
+            "active_ids": invoice.ids,
+        }
+        wizard = (
+            self.register_payments_model.with_user(self.user1)
+            .with_context(**ctx)
+            .create({"journal_id": self.cash2_journal_b2b.id})
+        )
+
+        available_journals = wizard.available_journal_ids
+        for journal in available_journals:
+            self.assertTrue(
+                not journal.operating_unit_id or journal.operating_unit_id == self.b2b,
+                f"Journal '{journal.name}' (OU={journal.operating_unit_id.name})"
+                f" should not be available for OU {self.b2b.name}",
+            )
+
+        self.assertNotIn(
+            self.cash_journal_ou1,
+            available_journals,
+            "OU1 journal should not be available when paying a B2B invoice",
+        )
+
+        wizard.action_create_payments()
diff --git a/account_operating_unit/tests/test_payment_register_journal_ou.py b/account_operating_unit/tests/test_payment_register_journal_ou.py
new file mode 100644
index 0000000000..c47f792f8a
--- /dev/null
+++ b/account_operating_unit/tests/test_payment_register_journal_ou.py
@@ -0,0 +1,151 @@
+# © 2026 BITVAX
+# License LGPL-3.0 or later (https://www.gnu.org/licenses/lgpl.html).
+"""Bug: account.payment.register wizard's available_journal_ids
+includes journals from operating units the invoice does not belong to.
+
+A B2B invoice should only offer journals whose operating_unit_id
+matches B2B (or no OU). On upstream/18.0 the wizard pulls journals via
+``account.journal.search()`` without OU context, so it returns every
+sale/cash journal of the company regardless of OU.
+
+The fix is to override
+``account.payment.register._get_batch_available_journals`` and filter
+the returned set by the operating_unit_id of the invoices in the
+batch.
+"""
+
+from odoo.models import Command
+from odoo.tests import tagged
+
+from odoo.addons.account.tests.common import AccountTestInvoicingCommon
+from odoo.addons.operating_unit.tests.common import OperatingUnitCommon
+
+
+@tagged("post_install", "-at_install")
+class TestPaymentRegisterJournalOu(AccountTestInvoicingCommon, OperatingUnitCommon):
+    @classmethod
+    def setUpClass(cls):
+        super().setUpClass()
+
+        (cls.ou1 | cls.b2b | cls.b2c).sudo().write({"company_id": cls.company.id})
+
+        cls.env.user.sudo().write(
+            {
+                "groups_id": [
+                    Command.link(
+                        cls.env.ref("operating_unit.group_manager_operating_unit").id
+                    ),
+                ],
+                "operating_unit_ids": [
+                    Command.link(cls.ou1.id),
+                    Command.link(cls.b2b.id),
+                ],
+                "default_operating_unit_id": cls.ou1.id,
+                "company_ids": [Command.link(cls.company.id)],
+                "company_id": cls.company.id,
+            }
+        )
+
+        cls.user1.write(
+            {
+                "groups_id": [
+                    (
+                        3,
+                        cls.env.ref("operating_unit.group_manager_operating_unit").id,
+                    ),
+                    Command.link(
+                        cls.env.ref("operating_unit.group_multi_operating_unit").id
+                    ),
+                    Command.link(cls.env.ref("account.group_account_invoice").id),
+                ],
+                "assigned_operating_unit_ids": [(6, 0, [cls.b2b.id])],
+                "default_operating_unit_id": cls.b2b.id,
+                "company_id": cls.company.id,
+                "company_ids": [Command.link(cls.company.id)],
+            }
+        )
+
+        Journal = cls.env["account.journal"].sudo()
+        cls.purchase_journal_b2b = Journal.create(
+            {
+                "name": "Vendor Bills B2B (test_pay_reg)",
+                "code": "TPRPB",
+                "type": "purchase",
+                "company_id": cls.company.id,
+                "operating_unit_id": cls.b2b.id,
+            }
+        )
+        cls.cash_journal_ou1 = Journal.create(
+            {
+                "name": "Cash OU1 (test_pay_reg)",
+                "code": "TPRC1",
+                "type": "cash",
+                "company_id": cls.company.id,
+                "operating_unit_id": cls.ou1.id,
+            }
+        )
+        cls.cash_journal_b2b = Journal.create(
+            {
+                "name": "Cash B2B (test_pay_reg)",
+                "code": "TPRCB",
+                "type": "cash",
+                "company_id": cls.company.id,
+                "operating_unit_id": cls.b2b.id,
+            }
+        )
+        cls.expense_account = cls.env["account.account"].search(
+            [
+                ("account_type", "=", "expense"),
+                ("company_ids", "in", cls.company.ids),
+            ],
+            limit=1,
+        )
+
+    def test_payment_register_filters_journals_by_invoice_ou(self):
+        invoice = (
+            self.env["account.move"]
+            .with_context(default_move_type="in_invoice")
+            .create(
+                {
+                    "partner_id": self.partner1.id,
+                    "operating_unit_id": self.b2b.id,
+                    "invoice_date": "2026-01-01",
+                    "journal_id": self.purchase_journal_b2b.id,
+                    "invoice_line_ids": [
+                        (
+                            0,
+                            0,
+                            {
+                                "name": "Line",
+                                "quantity": 1,
+                                "price_unit": 100.0,
+                                "account_id": self.expense_account.id,
+                                "tax_ids": [],
+                            },
+                        )
+                    ],
+                }
+            )
+        )
+        invoice.action_post()
+
+        wizard = (
+            self.env["account.payment.register"]
+            .with_user(self.user1)
+            .with_context(
+                active_model="account.move",
+                active_ids=invoice.ids,
+            )
+            .create({"journal_id": self.cash_journal_b2b.id})
+        )
+        available = wizard.available_journal_ids
+        self.assertIn(
+            self.cash_journal_b2b,
+            available,
+            "B2B cash journal must be available to pay a B2B invoice.",
+        )
+        self.assertNotIn(
+            self.cash_journal_ou1,
+            available,
+            "OU1 cash journal must NOT be available when paying a " "B2B invoice.",
+        )
diff --git a/account_operating_unit/wizard/__init__.py b/account_operating_unit/wizard/__init__.py
new file mode 100644
index 0000000000..019698a882
--- /dev/null
+++ b/account_operating_unit/wizard/__init__.py
@@ -0,0 +1 @@
+from . import account_payment_register
diff --git a/account_operating_unit/wizard/account_payment_register.py b/account_operating_unit/wizard/account_payment_register.py
new file mode 100644
index 0000000000..868feb8649
--- /dev/null
+++ b/account_operating_unit/wizard/account_payment_register.py
@@ -0,0 +1,21 @@
+# License LGPL-3.0 or later (https://www.gnu.org/licenses/lgpl.html).
+
+from odoo import api, models
+
+
+class AccountPaymentRegister(models.TransientModel):
+    _inherit = "account.payment.register"
+
+    @api.model
+    def _get_batch_available_journals(self, batch_result):
+        """Filter available journals by the operating unit of the invoices."""
+        journals = super()._get_batch_available_journals(batch_result)
+        lines = batch_result.get("lines")
+        if lines:
+            invoice_ous = lines.move_id.operating_unit_id
+            if invoice_ous and len(invoice_ous) == 1:
+                journals = journals.filtered(
+                    lambda j: not j.operating_unit_id
+                    or j.operating_unit_id == invoice_ous
+                )
+        return journals