[4] Oi (then known as Telemar) was formed as Tele Norte Leste to merge sixteen state-owned incumbent local exchange carriers, during the privatization of Brazilian telecommunications system.

The states that formed the base of Telemar at its inception were Rio de Janeiro, Minas Gerais, Espírito Santo, Bahia, Sergipe, Alagoas, Pernambuco, Paraíba, Rio Grande do Norte, Piauí, Ceará, Maranhão, Pará, Amazonas, Amapá and Roraima, corresponding to 65% of the Brazilian territory and 20 million households.

Today, Telemar and its subsidiaries offer local, long-distance and international voice and data services, besides a growing mobile phone network.

[7] Telemar Participações was incorporated by Oi, simplifying the corporate structure and terminating the shareholders' agreement.

[9][10] In February 2014, Oi announced it would raise $5.9 billion in a share offering as part of the firm's merger process with Portugal Telecom.

[14] In December 2020, a consortium formed by Vivo, TIM e Claro purchased the mobile assets for R$16.5 billion.

CADE (Administrative Council for Economic Defense) approved the purchase with reservations on January 9 of the same year.

Oi Internet started services with a promotion that offered 31% off the dial-up connection costs on the subscriber's bill.

Later, the ISP relaunched the promotion, offering 31% of the dial-up connection costs deposited in the subscriber's bank account or twice of that on a prepaid Oi phone.

In 2008, Oi announced it would purchase Brasil Telecom, creating a major Brazilian telecommunications company, already nicknamed "Supertele" or "SuperOi".