The Battle of Yu Oc (19 November 1884) was a French victory during the Sino-French War. The battle was fought to relieve the French garrison of Tuyên Quang, under siege by the Tang Jingsong's Yunnan Army and Liu Yongfu's Black Flag Army. The French commander at Yu Oc, Colonel Jacques Duchesne, would later distinguish himself as the conqueror of Madagascar (1895).

The French installed a post at Tuyên Quang in June 1884, in the wake of their capture of Hưng Hóa and Thái Nguyên. Tuyên Quang, an isolated settlement on the Clear River, was the most westerly French outpost in Tonkin, and was more than 100 kilometres (62 mi) away from the French garrisons in Hưng Hóa and Thái Nguyên. During the summer and autumn of 1884 the post was garrisoned by two companies of the 1st Battalion, 1st Foreign Legion Regiment (Captains Chmitelin and Broussier), under the command of chef de bataillon Frauger.

The outbreak of the Sino-French War on 23 August 1884 exposed the post to attack by Tang Jingsong's Yunnan Army and Liu Yongfu’s Black Flag Army. Supply difficulties delayed the Chinese concentration around Tuyen Quang, but advance elements of the Yunnan Army began to harass the post on 12 October 1884, and Frauger's garrison had to fight off a number of nuisance attacks by the Chinese between 13 and 19 October. Malaria had also taken a heavy toll of Frauger's men, and by the end of October 170 men out of the garrison's total strength of 550 men were unfit for duty.

During October and November 1884 the Farcy gunboats Revolver and Mitrailleuse, based at Tuyen Quang, and the gunboats Bourrasque, Éclair, Mutine and Trombe, operating out of Hưng Hóa, mounted a number of dangerous supply runs along the Clear River between Hung Hoa and Tuyen Quang in support of the small French garrison at Tuyen Quang. The first supply run, made by Revolver and Mitrailleuse on 15 October 1884, was successful, but at the end of October the Black Flags occupied Yu Oc, cutting off Tuyên Quang by land from the nearest French post at Hung Hoa and also commanding the river route to Tuyên Quang. A few days earlier the gunboat Mutine had been ordered to reinforce Revolver and Mitrailleuse at Tuyên Quang, but she ran aground during her voyage up the Clear River and Éclair had to be sent to pull her off and tow her back to Hung Hoa.

During the first half of November the French suffered a constant dribble of casualties as their gunboats sailing up and down the Clear River were engaged by the Black Flags at Yu Oc. In an engagement on 12 November Trombe suffered casualties of 1 man dead and 7 wounded. On one occasion Revolver had to steam at full speed to break a barrage laid across the Clear River, and on 16 November her crew sustained casualties of 2 men dead and 3 wounded (including the commander, enseigne de vaisseau de Balincourt) from enemy fire from Yu Oc.

The commander of the Tonkin Expeditionary Corps General Louis Brière de l'Isle, reacted swiftly to the attack on Revolver on 16 November. Foreseeing further such attacks, he decided to despatch an expedition immediately to clear the Black Flags away from Yu Oc. At the same time, a food convoy would leave by water by resupply and relieve the garrison of Tuyên Quang. A column of five infantry companies with supporting artillery was formed at Hung Hoa under the command of Lieutenant-Colonel Jacques Duchesne.

Duchesne's column set out for Tuyên Quang on the morning of 18 November. The French troops were ferried aboard a flotilla of junks, escorted by the four gunboats based at Hưng Hóa, to a point 7 kilometres (4.3 mi) above Yu Oc. The troops went ashore on the afternoon of 18 November on the right bank of the Clear River, and began marching slowly, in single file, towards Yu Oc and Tuyên Quang. By the evening of 18 November the column was within a few hours' march of Yu Oc. No sign of the enemy had yet been seen.

At dawn on 19 November the column set out for Yu Oc. Herbin and Chanu's marine infantry companies led the way. The artillery followed and the two Legion companies brought up the rear. The marsouins advanced in silence, but they knew that the Chinese were aware of their march. At around 7:00 am three shots were fired at the head of the column. The Chinese scouts were warning their comrades that the French were coming. The marine infantry continued to advance, waiting for the Chinese to show themselves. Minutes later the advance guard came under fire from three sides from Chinese skirmishers. The Chinese were deployed across the path, in the woods to its west, and on the further bank of the Clear River.