The structures of nitrogen-containing paramagnetic centers OK1 and N3 have not been unambiguously determined yet. In this paper, we will present data on the influence of high temperature annealing on the paramagnetic centers OK1 and N3 in diamond. A series of natural colored diamond crystals of cubic habit containing paramagnetic centers OK1 and N3 was studied by electron paramagnetic resonance (EPR), luminescence, and IR spectroscopies. It was found that annealing at temperatures above 2470 K led to the transformation of N3 paramagnetic centers into OK1 paramagnetic centers. Previous work has showed that a temperature in excess of 2400 K are often associated with a local environment change of tetrahedral to octahedral for impurities with large atomic radii. We report a correlation with the intensity of the optical 635.7 nm system and the concentration of the N3 EPR center. However, on stepped annealing 635.7 nm was removed before the N3 EPR center. From the analysis presented, we propose that the N3 EPR center and the 635.7 nm optical system are the manifestation of various defects formed by one impurity atom. 1 Introduction Although the nitrogen-containing paramagnetic centers OK1 and N3 [1, 2] were first detected more than 30 years ago, the identity of these centers was not been unambiguously determined [2][3][4][5]. Regarding the nature of defects in diamond, more than 500 have been characterized with only a handful identified. Of those positively identified, EPR was often a key tool used in unlocking their structure and identity. This includes nitrogen centers P1 [6], P2 [7,8], W7 formed by the interaction of Acenters with the dislocation [9,10], and N 2 V which is the result of the interaction of A-centers with vacancies [10]. In addition, the structures of nickel-nitrogen centers NE1-NE9 [11][12][13][14], boron-nickel centers [15], nitrogen-cobalt centers [16,17], nitrogen-phosphorus centers NP1-NP7 [18][19][20], oxygen-containing [21], and hydrogen-containing centers [22,23] were determined. OK1 and N3 EPR centers remain unidentified but are often observed together, as shown by recent studies [23]. They are a characteristic