Ouvrage Billig, Maginot Line artillery ouvrage in Oudrenne, Moselle, France.

Ouvrage Billig is an underground military fortification in eastern France with multiple combat blocks linked by tunnel passages. The complex spans several levels and contains areas for barracks, food preparation, medical care, and storage of water and supplies.

Approved by CORF in June 1930 and operational by 1935 at a cost of 65 million francs, Billig was built by contractor Ossude of Paris. Its 537-man garrison, including 16 officers under Commandant Roy, fired its first shots on 12 May 1940. On 15 June an aerial bomb penetrated the Block 6 mortar turret, killing two; on 24 June Billig fired 2,030 75 mm shells in support of Hackenberg. The ouvrage was handed over intact to the Germans on 4 July 1940.

The ouvrage was manned in 1940 by 537 men and 16 officers of the 167th Fortress Infantry Regiment and the 151st Position Artillery Regiment, under Commandant Roy. The underground casernement at Elzange supplied peacetime barracks and support services for Billig and neighbouring ouvrages. The single mixed entrance, used for both ammunition and personnel, is one of only two true mixed entries in the northeast, with Vélosnes and Chesnois.

The ouvrage stands on the commune of Oudrenne, between the gros ouvrages of Métrich and Hackenberg. It is abandoned and vandalised, and its openings have been closed off to prevent access, so the interior galleries cannot be entered; the above-ground blocks and their rust-framed embrasures remain visible.

The fortification uses an unusual feature: a shared entrance for ammunition and personnel that differs from other comparable defensive structures. This compact arrangement reveals a different approach to designing underground military facilities.