2010: Oh!/Run Devil Run, Japanese debut and Hoot

Later in January, SM Entertainment confirmed the release of Oh!, the group's second full-length album, to be on January 28. The lead single "Oh!" was digitally released on January 25 with the music video being released two days later. On January 30, the group started promotions with the recording of MBC Music Core.The broadcast of the performance experienced a blackout near the end, to which MBC made the whole performance available online in response. With "Oh!", the group went on to win five consecutive K-charts on KBS Music Bank, as well as achieving a triple crown on SBS Inkigayo. "Oh!" also went on to win the Music Bank half-year K-chart, coming in at second on Hanteo's first half album sales chart,and also winning the end-of-year K-chart. The music video ranked as the number one most viewed YouTubevideo in Korea by the end of 2010. It was also number one in Hong Kong and made the top ten in both the Taiwan and Japan rankings.

Starting on March 11, 2010, photos of the members were released online showcasing a dark concept, dubbed "Black SoShi". A teaser video was released on March 16, with the new single Run Devil Run being released as a digital single on March 17.

Mid-June, it was announced that the group had signed contracts in May, and would begin working under Universal Music Japan's record label Nayutawave Records for their Japanese promotions, with their first Japanese single expected to be released in September 2010. Girls' Generation began promotion in Japan in August. "少女時代到来～初来日記念盤～" (Girls' Generation's Arrival ～ First Time in Japan Commemoration Disc ～), a DVD featuring seven of the group's music videos as well as special bonus footage, was released on August 11, with the special edition of the DVD containing a pink glowstick as well as a pass to the Girls' Generation debut at the Tokyo Ariake Colliseum on August 25. In the first week after release, the DVD sold 23,000 copies, ranking fourth on the weekly DVD Oricon ranking and third on the weekly music DVD ranking,and made Girls' Generation the first female K-pop group to make the Oricon DVD Top-5 ranking.

Amidst their Japanese activities, the group also participated in the SMTown Live '10 World Tour alongside their label-mates on August 21 at Seoul Jamsil Olympic Stadium.They participated in the subsequent Shanghai, Los Angeles, Paris and Tokyo stops as well.

On August 25, the group held their first showcase in Tokyo Ariake Colliseum.With an initial 10,000 fans invited, it was reported to be the most large-scale plan for a Korean artist debuting in Japan. However, due to high number of expected attendees, the group's management announced that the showcase would take place three times in the same day rather than once to accommodate the large amount of fans. The group sang five of their Korean tracks at each of the showcases, and the total number of attendees over all three shows were estimated to be at least 22,000. At this showcase, the "Genie" Japanese music video was also revealed publicly for the first time and the teaser having been released a few days prior on August 20.

Their Japanese debut single "Genie" debuted at the fifth spot on the same day of its release on the Daily Oricon Charts and later rose to the second position on the Daily Charts some time later, earning the group a fourth position on the Oricon weekly ranking. The group also hit the number one spots on Japan iTunes' music video chart and both the Rekochoku daily video clip chart and incoming movie daily ranking chart a day later.^[51] With pre-orders amounting to 80,000, the single sold an estimated 45,000 copies in the first week after release. On October 20, 2010, the group released their second Japanese single, "Gee".A week after the release of their Japanese single, the group released their third Korean mini-album, Hoot on October 27, 2010, consisting of five tracks. The video for lead single, "Hoot", a song originally written in English as "Bulletproof", was released on October 28. Girls' Generation commenced their promotional activities with a comeback stage on KBS Music Bank on October 29. "Hoot" quickly rose to the top of the charts and received the number one spot numerous times on music shows, proving to be another successful release for the group. The group simultaneously promoted "Hoot" and "Gee" in Korea and Japan respectively, and participated in Japanese music broadcasts such as the FNS Music Festival, where they performed their two Japanese singles, "Gee" and "Genie" on December 4.

On December 22, 2010, "Hoot" was repackaged with additional tracks from previous albums and released in Japan. The release rose to the second position on the Oricon charts and sold 21,000 copies on the first day, faring surprisingly well considering that all the included tracks were entirely in Korean.

On December 9, 2010, the group attended the Golden Disk Awards and was awarded three awards including the Disk Daesang (Album of the Year) for their 2nd album Oh!. The group also became the first girl group to have both a Disk Daesang (2010) and Digital Daesang (2009). On December 15, 2010, the group attended the Melon Music Awards winning the Best Dressed Singer award and Hot Trend Song award for, "Hoot", as well as the Artist of the Year award. This would be the second consecutive 'Artist of the Year' award for the group. They topped the Hanteo annual Singer Award chart for 2010.

The group was named on Asia Today's list of 50 Korean power leaders for 2011 at the number forty-fourth spot, being the only idol groupfrom the list. Girls' Generation was designated "Artist of the Year" by Dosirak, Soribada, Korea Gallup, Hanteo and Sport Korea and also won Best Song of The Year with "Oh!" at Monkey 3.