In order to use FIFO, LIFO, or FEFO you first have to activate and configure your Traceability, Location, and Multi-Step Route  settings under Inventory-> Configuration. 

If these settings are not configured there is no particular order in which products are moved, because as far as Odoo is concerned you are not wanting to track the products. In this case the stock level you have is just a number, Odoo doesn't know which items were received first or last. 

I hope this helped!