Uom in opportunity line is updated when creating the sale
On the sale_opportunity lines, there's no domain on the unit field allowing you to select any unit for a product.
But when you create a sale for this opportunity, the unit on the sale line will be the product sale unit so your sale could be different of your opportunity if the unit choosen in the opportunity is not the sale unit.
I mean product A has 'unit' as sale unit but you make an opportunity with product A and 'Kg' as unit and quantity:10. When you create the sale, product 'A' will get 'unit' as uom instead of 'Kg' because 'unit' is the sale_unit. But 10 Kg or 10 unit is totally different...