Until 2013, OU Kosher did not permit quinoa for Pesach. The following is what the OU released about quinoa in the past: (http://oukosher.org/passover/guidelines/food-items/quinoa/)

“There is a difference of opinion among Rabbinic decisors (Machloket Ha-Poskim) as to whether quinoa is considered kitniyot. Ask your Rabbi for his guidance. Additionally, while quinoa is not one of the five grains that can create chometz (wheat, oat, barley, spelt, and rye), and quinoa is not grown in the same vicinity as the grains mentioned above, the processing of quinoa is sometimes done at the same location where they process wheat and wheat flour. It is highly doubtful that the mills are effectively cleaned between grains. The concern of wheat flour or particles finding their way into the quinoa flour would be a serious one”.

In the OU Pre-Pesach webcast of March 12, 2013, both Rav Belsky (stringent) and Rav Schachter (lenient) presented their respective views on this matter. Rav Belsky argues that quinoa fits every criterion for kitniyot and should be included in its prohibition. Quinoa is the staple grain in its country of origin. It is grown in proximity to and can be mixed up with the five grains. It is collected and processed the same (and in the same facilities) as the five grains and is cooked into porridge and breads the same way. He maintained that we should compare quinoa to corn, which was, for similar reasons, defined as kitniyot.  

However, in late 2013, OU Kosher changed its policy and permitted quinoa for Pesach. The OU joined the consensus of major American kashrut organizations, the Star-K (following Rav Moshe Heinemann), Kof-K, OK, and CRC (following Rav Gedalia Dov Schwartz), in permitting quinoa. This consensus is surprising in light of the general trend towards stringency regarding Pesach, including kitniyot. The following cases are prime examples of this phenomenon.  

Kitniyot Oils and Syrups - Rav Kook and the Badatz 

In the year 5669 (1909), a great controversy erupted between Rav Avraham Yitzchak Kook, then chief rabbi of Jaffa, and the Jerusalem Badatz regarding the permissibility of sesame seed oil on Pesach. Rav Kook published a short work (summarized in Yesodei Yeshurun 6:424 and Hamo'adim Bahalachah, p.259) outlining why he permitted this item for Pesach consumption.

Although sesame seeds are kitniyot, he notes that their form has changed into oil. No oil can ever become chametz even if it contacts water, so the minhag to avoid kitniyot does not include sesame seed oil. Similarly, the concern of confusing kitniyot with grain does not apply once the sesame seeds are no longer in grain form. Furthermore, Rav Kook addresses sesame seeds that were watched to ensure they would not be exposed to water. The machine producing the oil needed to be completely dry to function, further guaranteeing no concern for chametz. Finally, the sesame seeds in question were cooked during their processing, and once something is cooked, it can no longer become chametz. Thus, the sesame seed oil in question was so far removed from a concern of chametz that, according to Rav Kook, the minhag of kitniyot did not apply to it. 

The Jerusalem Badatz strongly rejected Rav Kook's lenient ruling. They argued that the Rama (Orach Chaim 453:1) clearly includes oils derived from kitniyot in the custom of avoiding kitniyot, as noted by the Chayei Adam (Nishmat Adam 33). 

Rav Kook responded that the Rama only intends to prohibit oil from kitniyot that was not carefully inspected for other grains or oil from kitniyot that were not cooked or boiled. Several of his contemporaries accepted Rav Kook's understanding of the Rama and his subsequent ruling, including the Maharsham (Teshuvot 1:183), the Marcheshet (Chapter 3), Rav Yitzchak Elchanan Spektor (Teshuvot Be'er Yitzchak, O.C. 11) and Rav Shamshon Raphael Hirsch, cited in Teshuvot Melamed L’Ho’il O.C. 88). Some authorities permit oil of products whose kitniyot status is questionable, such as peanut oil (Rav Tzvi Pesach Frank's Mikra'ei Kodesh p.205-206 and Teshuvot Chelkat Yaakov 1:97).

Despite the cogency of Rav Kook's arguments and the many great authorities that support them, common practice prohibits kitniyot oil. For example, Coca-Cola changes its formula from corn syrup to sugar for its kosher-for-Pesach colas. 

Kitniyot Shenishtanu 

Today, many food products contain kitniyot that have been manufactured and metamorphosed into dramatically different and new products, such as ascorbic acid, citric acid, lactic acid, NutraSweet, and MSG. Aspartame is a commonly used product made from kitniyot shenishtanu. Rav Hershel Schachter (B'ikvei Hatzon 27:9) and Rav Yisroel Belsky permit transformed kitniyot for Pesach use. They base their ruling on the Mishna Brura 216:7, which states that we may be lenient about a dramatically changed product that involves only a rabbinic prohibition. Since kitniyot is only a custom, one certainly may be lenient.    

By contrast, the Star-K does not permit them in Pesach foods. The Star-K writes:

When Rabbi Moshe Heinemann, shlit”a, Rabbinic Administrator of the STAR-K, discussed this issue with Rabbi Yosef Shalom Eliyashiv, zt”l, and Rabbi Shlomo Zalman Auerbach, zt”l, their position was to prohibit kitniyot shenishtanu as a Chumra d’Pischa, a strict adherence to the minhag of prohibiting kitniyot. For this reason, it is STAR-K's policy not to certify products containing kitniyot shenishtanu.

No consensus has emerged regarding kitniyot shenishtanu, and the major Hashgacha granting organizations maintain differing standards.