The present invention relates to a barrier testing method for testing a production casing in a borehole (4). The method is applied before initiating production in a well and comprises the steps of connecting a drill pipe (10) with a first end (20) of a first production casing having annular barriers (17), which annular barriers (17) comprise a tubular part forming part of the casing and an expandable sleeve circumferenting the tubular part, thereby defining an expandable space; inserting the drill pipe (10) and the first production casing (3) via a drill head (6) arranged at a top (7) of the well into an intermediate casing (11) extending in a first part (18) of the borehole (4) closest to the top of the well and at least part of the first production casing into a second part (19) of the borehole; sealing a second end (21) of the first production casing (3); pressurizing the first production casing (3) from within and expanding one or more of the expandable sleeves (22) of the annular barriers (17) to abut a wall of the borehole; pressurizing the first production casing (3) from within to a predetermined pressure; and testing the first production casing (3) after expansion by measuring if the predetermined pressure is kept constant during a predetermined time period. Furthermore, the invention relates to a completion system for oil production from a well and to an oil production facilitated by the method barrier testing method.