It should be many to many,
as 1 product say “Dell Laptop” could be in the order created by harsh as well other users.
at the same time, 1 order can have many items.
Hope i got the context :)
As Ganesh said, it should be many to many.
An order could contain:
1x Biryani
1x Dosa
but Dosa could be ordered by each customer so it could be in each order.
so 1 order could have multiple order_item and 1 order_item could belong to multiple orders.
I hope it clears your doubt.