Oreo (/ˈɔːrioʊ/; stylized in all caps) is a brand of sandwich cookie consisting of two cocoa biscuits or cookie pieces with a sweet fondant filling. It was introduced by Nabisco on March 6, 1912, and through a series of corporate acquisitions, mergers, and splits, both Nabisco and the Oreo brand have been owned by Mondelez International since 2012. Oreo cookies are available in over one hundred countries. Many varieties of Oreo cookies have been produced, and limited-edition runs have become popular in the 21st century.

Oreos are an imitation of the Hydrox chocolate cream-centered cookie introduced in 1908, but it outstripped Hydrox in popularity so largely that many believe Hydrox is an imitation of Oreo. Oreo has been the highest-selling cookie brand in the world since 2014.

The origin of the name "Oreo" is unknown, but there are many hypotheses, including derivations from the French word or, meaning "gold" (the original tin was gold-colored); the Greek word ὄρος (oros), meaning "mountain" (the cookie was originally conceived to be dome-shaped); or the Greek word ωραίο (oreo) meaning "nice" or "attractive". Others believe that the cookie was named Oreo simply because the name was short and easy to pronounce. Another theory, proposed by the food writer Stella Parks, is that the name derives from the Greek Oreodaphne, a genus of the laurel family, originating from the Greek words 'oreo' (ωραίο) meaning 'beautiful' and 'daphne' (δάφνη) referring to the laurel. She observes that the original design of the Oreo includes a laurel wreath, and the names of several of Nabisco's cookies at the time of the original Oreo had botanical derivations, including Avena, Lotus, and Helicon (from Heliconia).

The "Oreo Biscuit" was first developed and produced by the National Biscuit Company (today known as Nabisco) in 1912 at its Chelsea, New York City factory in the present-day Chelsea Market complex, located on Ninth Avenue between 15th and 16th Streets. In 2002, this same block of Ninth Avenue was ceremoniously named as "Oreo Way". The name Oreo was trademarked on March 14, 1912. It was launched as an imitation of the original Hydrox cookie manufactured by Sunshine company, which was introduced in 1908.

The original design on the face of the Oreo featured a wreath around the edge of the cookie and the name "OREO" in the center. In the United States, they were sold for $0.25 (equivalent to $7.89 in 2023) a pound (454 g) in novelty metal canisters with clear glass tops. The first Oreo was sold on March 6, 1912, to a grocer in Hoboken, New Jersey.

The Oreo Biscuit was renamed in 1921 to "Oreo Sandwich"; in 1948, the name was changed to "Oreo Crème Sandwich"; and in 1974 it became the "Oreo Chocolate Sandwich Cookie", the name that has remained to this day. A new design for the face of the cookie was launched in 1924; the modern-day Oreo design was developed in 1952 by William A. Turnier, incorporating the Nabisco logo. In 1920, a second lemon crème-filled variety of the Oreo was introduced, as an alternative to the white crème-filled variety, but this was discontinued in 1924 and the original flavor was the only version available for the next several decades.

The modern Oreo cookie filling was developed by Nabisco's principal food scientist, Sam Porcello, who retired from Nabisco in 1993. Porcello held five patents directly related to his work on the Oreo; he also created a range of Oreo cookies that were covered in dark chocolate and white chocolate. In the early 1990s, health concerns prompted Nabisco to replace the lard in the crème filling with partially hydrogenated vegetable oil. This also made it possible for the bakery to become kosher-certified, a lengthy and expensive process for Nabisco. Similarly, Oreo cookies are popular with people that have certain dietary restrictions, such as vegans, as the crème filling does not use any animal products. However, there is still a risk of cross-contamination from other dairy-containing products made in the same production areas; in the FAQ section of the Oreo website, on the question of whether Oreo cookies are suitable for vegans, the response is "Many Oreo products are suitable for vegans but may include cross-contaminants of milk, so please check allergen advice."

In January 2006, Nabisco and its parent (at the time) Kraft Foods eliminated the trans fat in the Oreo cookie and replaced it with non-hydrogenated vegetable oil as one of the main ingredients for Oreo cookies in general.

In June 2012, Oreo posted an advertisement displaying an Oreo cookie with rainbow-colored crème to celebrate LGBT Pride month; the cookie itself was fictional and was not being manufactured or made available for sale. The advertisement prompted some negative comments from conservatives, but Kraft stood by their promotion, stating that "Kraft Foods has a proud history of celebrating diversity and inclusiveness. We feel the Oreo ad is a fun reflection of our values." This was followed during 2012 by a series of adverts commemorating other holidays and events, including a blue, white, and red crème Oreo to honor Bastille Day, a stream of cookie crumbs marking the appearance of the Delta Aquariids meteor shower, and a cookie with a jagged bite taken out of it to promote Shark Week on Discovery Channel.

When the power went out during Super Bowl XLVII in 2013, the Oreo marketing team tweeted "you can still dunk in the dark" from its social media command center; this was retweeted almost 15,000 times and increased Oreo's count of Twitter, Facebook, and Instagram followers. One commentator remarked that this "solidified the viability and necessity of real-time marketing".

According to an April 2022 research report published in the journal Physics of Fluids, it was proven impossible to split the cream filling of an Oreo cookie down the middle. The filling always adheres to one side of the wafer, no matter how quickly the cookie is twisted.

Oreo cookies are distributed worldwide through a variety of sales and marketing channels. As their popularity continues to grow, so too does the amount of distribution that comes with it. According to the Kraft Foods company, the Oreo is the "World's Best Selling Cookie". In March 2012, Time magazine reported that Oreo cookies were available in more than 100 countries. Overall, it is estimated that since the Oreo cookie's inception in 1912, over 450 billion Oreos have been produced worldwide.

Oreos were first introduced into Britain through the supermarket chain Sainsbury's. For several years, this was the only supermarket chain in the UK to stock the Oreo until May 2008, when Kraft decided to fully launch the Oreo across the whole of the UK. Its packaging was redesigned into the more familiar British tube design, accompanied by a £4.5M television advertising campaign based around the "twist, lick, dunk" catchphrase. In a 2020 national poll the Oreo was ranked the 16th most popular biscuit in the UK, with McVitie's chocolate digestive topping the list.

In the UK, Kraft partnered with McDonald's to introduce the Oreo McFlurry (which was already on sale in several other countries, including the US) into McDonald's locations across the country during its annual Great Tastes of America promotions; in October 2015, the Oreo McFlurry then became a permanent menu item at McDonald's in the UK. An Oreo-flavored "Krushem" drink was also on sale in KFC stores across Britain.

The ingredients of the British Oreo (as listed on the UK Oreo website) are slightly different from those of the US Oreo. Unlike the US version, the British Oreo originally contained whey powder, which was not suitable for people with lactose intolerance. Additionally, as the whey powder was sourced from cheese made with calf rennet, the British version was also unsuitable for vegetarians. On December 6, 2011, Kraft announced that production of Oreo was to start in the UK with their Cadbury Trebor Bassett factory in Sheffield, South Yorkshire, being selected to manufacture Oreo in Britain for the first time. Production began there in May 2013.

Oreo cookies were introduced onto the Indian market by Cadbury India in 2011. In Pakistan, Oreo is manufactured and sold by Continental Biscuits Limited under the LU brand. In Japan, Oreo and other Nabisco products were produced by Yamazaki Baking until Mondelez terminated their licensing deal in favor of moving production to China. A year later, Yamazaki introduced their version of Oreo called "Noir", which is produced at the former Oreo factory in Ibaraki Prefecture.

Most of the Oreo production was once carried out at the Hershey's factory in Hershey, Pennsylvania. By 2017, more than 40 billion Oreo cookies were being produced annually in 18 countries around the world. Oreo cookies for the Asian markets are manufactured in India, Indonesia, Bahrain, and China. Oreo cookies for the European market are made in Spain and at the Cadbury factory in the UK; they are made in Russia (Mondelēz Rus) for consumers in several CIS countries; and those sold in Australia are manufactured in Indonesia, China or Bahrain, depending on the flavor. The version produced in Canada (sold under the Christie's brand) included coconut oil but as of 2023, the ingredient list included vegetable oil and modified palm oil, similar to the American cookies. Manufacture of Oreo biscuits began in Pakistan in early 2014, in collaboration with Mondelez International of the United States and Continental Biscuits Limited (CBL) of Pakistan, at the CBL production plant in Sukkur.

In 2015, Mondelez announced its decision to close some of its American factories and move production to Mexico, prompting the Oreo boycott. In 2016, after production had started in Mexico, the AFL–CIO encouraged the boycott and published consumer guidance to help identify which Mondelez products were made in Mexico.

The ingredients of Oreo cookies have remained largely unchanged from the original, although numerous alternative varieties and flavors have emerged over time. Oreo cookies were made with lard until the mid-1990s, when Nabisco swapped the animal fat with partially hydrogenated vegetable oil. Then in the mid-2000s Nabisco eliminated the partially hydrogenated oil from the ingredients, due to growing health concerns. The classic Oreo cookie is made using eleven main ingredients:

1. Sugar
2. Unbleached enriched flour (wheat flour, niacin, iron, thiamine mono-nitrate (vitamin B₁), riboflavin (vitamin B₂), folic acid)
3. High oleic canola oil or palm oil
4. Cocoa (treated with alkali)
5. High-fructose corn syrup
6. Leavening agent (baking soda or monocalcium phosphate)
7. Corn starch
8. Salt
9. Soy lecithin
10. Vanillin
11. Chocolate