Novel trispirocyclotriphosphazenes with oxaphosphorine rings (DOP-PZs) were successfully synthesized by an Appel reaction with phosphoramide, which was prepared from ammonia and 10-chloro-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide derived from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, generally abbreviated as DOPO. The resulting DOP-PZs were characterized by 1H, 13C{1H}, and 31P{1H} nuclear magnetic resonance spectroscopy and time-of-flight mass spectrometry and shown to consist of cis-trans isomers. Moreover, the crystal and molecular structures of the DOP-PZs were determined by X-ray diffraction; cis- and trans-DOP-PZs (C36H24N3O3P3, M = 639.49 g/mol) were refined to final R1 values of 0.0260 and 0.0463, respectively, with the SHELXL refinement package using least-squares minimization. The crystal of cis-DOP-PZ is trigonal in space group R3c and the following cell constants: a = 19.5984(5) Å, c = 13.2754(4) Å, V = 4415.9(3) Å3, Z = 6, and Flack parameter = 0.038(8). In contrast, trans-DOP-PZ is monoclinic in space group P21/c and the following cell constants: a = 9.98647(18) Å, b = 24.1737(4) Å, c = 12.8472(2) Å, β = 112.649(8)°, V = 2862.26(18) Å3, and Z = 4. The molecular structures of these DOP-PZs were compared with those of other trispirocyclotriphosphazenes. In addition, the DOP-PZs showed high thermal stability up to 400 °C, with dielectric constants of 2.76-2.77 and dissipation factors of 0.0017-0.0031 at 10 GHz.