Ōzeki Takamasu served as a bushō and daimyō during the Sengoku and Azuchi-Momoyama periods.

In 1527, Takamasu was born as the eldest son of Ōtawara Sukekiyo, an influential retainer of the Nasu clan in Shimotsuke Province.

Around 1542, through the political maneuvers of his father, Takamasu became the adopted heir of Ōzeki Munemasu, inherited the headship of the clan, and became the lord of Shirahata Castle.  In 1549, Takamasu served valorously in his first experience in war at the Battle of Kitsuregawa-Sōtomezaka against the Shimotsuke-Utsunomiya clan.  In 1551, Takamasu was invested with the court titles of Junior Fifth Rank (Lower) and Assistant Captain of Outer Palace Guards of the Right Division.  Later, he received the title of Governor of Mimasaka.

In the third month of 1560, at the Battle of Odakura, during an invasion by the allied forces of the Shirakawa-Yūki and the Ashina clans, Takamasu supported Nasu Suketane and sought to defend against the attack.  After the battle, Suketane censured Takamasu and Ōtawara Tsunakiyo (siblings who were his retainers), assigning responsibility to them and triggering a confrontation with the Ōzeki and Ōtawara clans.  Takamasu responded by taking members of the Upper Nasu family and abandoning Suketane and colluding with the Satake clan of Hitachi Province.  On 5/15, Suketane encouraged Matsumoto Michikatsu, a retainer of Takamasu, to plot his assassination.  When Takamasu learned via Satake Yoshiaki of the plot, he had Okino Yoshishige guard him.  To expel Suketane, Takamasu made plans to back Satake Yoshihisa (later known as Nasu Suketsuna), the younger brother of Satake Yoshishige.

From 1563 to 1567, Suketane engaged in repeated battles against Takamasu (who obtained reinforcements from Yoshishige).  This included attacks below Karasuyama Castle, but Suketane repelled them.  On 8/24 of Eiroku 9 (1566), Takamasu joined the Satake Yoshikata and the Shimotsuke-Utsunomiya clan to attack Suketane, but Suketane routed them at Kaminaga in Shimotsuke Province.  In 1567, Takamasu and Satake Yoshishige fought against the Nasu clan at 大崖山 but were repelled.  Having lost in all of their battles against Suketane, in 1568, Takamasu and Tsunakiyo settled on the condition that Suketane retire while Takamasu underwent the rites of tonsure and apologized for his crimes of resistance.  Thereafter, Takamasu served as a leading figure among the Seven Clans of the Nasu in support of the Nasu clan.  In the era of Nasu Sukeharu, he wielded the most authority in the Nasu clan.  In 1576, Takamasu moved his base from Shirahata Castle to Kurobane Castle.  In 1578, he transferred the headship of the clan to his second son, Ōzeki Kiyomasu but, nevertheless, continued to maintain his grip on power.

In the third month of 1585, Takamasu, together with his lord, Nasu Sukekiyo, defeated the allied forces of Shionoya Yoshitsuna and Utsunomiya Kunitsuna at the Battle of Usubagahara during which the Ōzeki forces garnered many enemy heads.  In the twelfth month, Takamasu obtained consent from his lord, Sukeharu, to murder Senbon Suketoshi and Senbon Sukemasa (father and son).  He allocated a portion of the former territory of the Senbon clan to his younger brothers, Fukuhara Suketaka and Ōtawara Tsunakiyo.  He then arranged for Senbon Yoshitaka, the adopted son of Suketoshi who had returned to his original home with the Motegi clan, inherit the headship of the Senbon clan.  From the latter part of 1585 to the first month of 1586, he obtained consent from Sukeharu for his second son, Ōzeki Kiyomasu, to invade the territory of Iōno Sukenobu.  Owing to a victory by Kiyomasu, as a condition of settlement, he had the Iōno cede eastern portions of their territory, demonstrating actions taken under his own authority.

In 1590, during the Conquest of Odawara, the Nasu clan was late to deploy and, as a consequence, the clan was removed from its position.  In contrast, Takamasu quickly deployed and, for his service, received official recognition of his rights to landholdings of 10,000 koku.  Moreover, his eldest son, Ōzeki Harumasu, was granted 3,000 koku.  As a result, the Ōzeki clan controlled 13,000 koku in total attaining the status of a daimyō family.

According to a genealogical record, Takamasu died on 1/14 of Keichō 5 (1600), but in another record this was later modified to 1/14 of Keichō 3 (1598).  He was seventy-two years old.