The sandwich cookie introduced by Sunshine Biscuits in 1908, predating Oreo, was Hydrox.
Hydrox debuted in the United States in 1908 and was made by Sunshine Biscuits, an American bakery founded as the Loose-Wiles Biscuit Company. Its name was inspired by hydrogen and oxygen, elements associated with water and the ideas of purity and goodness.
Oreo arrived in 1912, four years later, and was created as a similar chocolate-and-creme sandwich cookie. Oreo eventually became much more popular, which led many people to assume that Hydrox copied it. Historically, the order was the reverse: Hydrox came first.
Hydrox has changed ownership several times. Sunshine was acquired by Keebler in 1996, and the cookie was later discontinued and revived. Leaf Brands reintroduced Hydrox in 2015. Compared with Oreo, Hydrox is often described as having a crunchier chocolate shell and a less-sweet filling.