With an Off Peak ticket on c2c services, you can travel on any Off Peak train for the date(s) shown on your ticket. With c2c, that means the ticket is generally valid for arrival into London after 10am on weekdays. For travel out of London, the ticket is valid for travel after 09:30am on weekdays. Generally, you can’t travel on weekday morning peak services with an Off Peak ticket.
There are some exceptions and other operators vary, so use the journey planner to check what the applicable fares are, depending on the time and day of travel.