Haplogroup O2a (Y-DNA)
In genetics, Haplogroup O2a (M95) is a human Y-chromosome DNA haplogroup.  Haplogroup O2a is a descendent branch of Haplogroup O2. Its closest extant phylogenetic relatives are the Haplogroup O2- Y-chromosomes found at a low frequency throughout most of East Eurasia and the Haplogroup O2b Y-chromosomes found at low frequencies among the indigenous populations of Inner Mongolia and Manchuria and at much higher frequencies in Korea and Japan.
Haplogroup O2a is distributed widely in Asia, from southern India to the Altai Mountains and Central Asia in the west, and from Indonesia to northern China and Japan in the east. It is found only at marginally low frequencies of approximately 1% at the periphery of its distribution in southern India, Central Asia, northern China, and Japan, but many populations within the vast intervening territory in South Asia, Southeast Asia, and southern China display a greatly elevated frequency of Haplogroup O2a Y-chromosomes. Patrilines within Haplogroup O2a predominate among the Austro-Asiatic populations of South and Southeast Asia, such as the Khmer of Cambodia and the Khasi of Meghalaya in northeastern India. Some researchers have reported that slightly over half of all men in a composite sample of Austro-Asiatic speakers belonged to Haplogroup O2a. Haplogroup O3 (M122), which attains its peak frequency among the Sino-Tibetan and Hmong-Mien peoples of China and Southeast Asia, and Haplogroup O1a (M119), which predominates among Taiwanese aborigines and many populations of the Philippines, also generally occur among speakers of Austro-Asiatic languages in South China and the Indochinese Peninsula, but at much lower frequencies than Haplogroup O2a. The hypothesis that Haplogroup O2a was the major Y-chromosome haplogroup of the proto-Austro-Asiatic population is strengthened by the fact that Haplogroup O2a is the only haplogroup found among many Austro-Asiatic-speaking tribes, such as the Juang of mainland India and the Shompen of the Nicobar Islands.
Haplogroup O2a has also been found to occur at a rather high frequency among the Daic-speaking peoples of Thailand and neighboring areas, which may reflect assimilation of the older Austro-Asiatic Mon-Khmer populations that have left ample evidence of their presence in the region prior to the immigration of Daic speakers.
Outside of the region in which Austro-Asiatic languages are currently spoken or have a historically attested presence, Haplogroup O2a reaches its highest frequencies among the populations of the islands of Sumatra, Java, Bali, and Borneo in western and central Indonesia. Haplogroup O2a has been found to be by far the most common Y-chromosome haplogroup among the Balinese, occurring in approximately 58.6% (323/551) of a sample of Balinese men; Haplogroup O1a-M119 and Haplogroup O3-M122, which are typical of Austronesian peoples outside of Malaysia and Indonesia, were observed in only 18.1% (100/551) and 6.9% (38/551) of Balinese men. Haplogroup O2a has also been found to be the most frequently occurring haplogroup among Malay men in Singapore. The reason for its substantial presence in these populations, all of which are Austronesian-speaking, is yet to be elucidated.

# Subgroups
The subclades of Haplogroup O2a with their defining mutation, according to the 2006 ISOGG tree:
- O2a (M95) Typical of Austro-Asiatic peoples, Tai-Kadai peoples, Malays, Indonesians, and Malagasy, with a moderate distribution throughout South Asia, Southeast Asia, East Asia, and Central Asia
O2a*
O2a1 (M88, M111) Frequently found among Hani, Tai peoples, and Cambodians, with a moderate distribution among Qiang, Hlai, Yao, Taiwanese aborigines, populations of Borneo, and Han Chinese of Sichuan, Guangxi, and Guangdong
O2a1*
O2a1a (PK4) Found at low frequency among Pashtuns
O2a2 (M297)
- O2a*
- O2a1 (M88, M111) Frequently found among Hani, Tai peoples, and Cambodians, with a moderate distribution among Qiang, Hlai, Yao, Taiwanese aborigines, populations of Borneo, and Han Chinese of Sichuan, Guangxi, and Guangdong
O2a1*
O2a1a (PK4) Found at low frequency among Pashtuns
- O2a1*
- O2a1a (PK4) Found at low frequency among Pashtuns
- O2a2 (M297)