We measured positron annihilation lifetime (PAL) spectra in N(2)O in the range from 3 nm(-3) to 13 nm(-3), focusing on density dependence of PAL in the medium including the supercritical fluid phase (critical temperature T(c) = 309.6 K, critical pressure P(c) = 7.24 MPa, critical density rho(c) = 6.18 nm-3). The PAL spectra were resolved into four components using the PATFIT code. We found the formation of two types of o-Ps. Namely, in addition to the longest lifetime component attributed to the o-Ps formation through the process explained by the Ore model, the third lifetime component (the second longest) was found in the density larger than 4.5 nm(-3), while the former was observed continuously from the gas phase to the liquid phase.