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This paper provides details of the potential designs of a high-pressure high-temperature (HPHT) sensor which forms part of a highly accurate optical measurement system for permanent monitoring of pressure and temperature in oil wells. The proposed sensors are of a hybrid construction and comprise two different types of spectrally encoded transducers - a fiber Bragg grating (FBG) and a Fabry-Perot (FP) cavity, which are connected in series. The FBG and FP sensors are used primarily to measure temperature and pressure, respectively. Because of the two different applications currently addressed - oil-sand production and deep-well oil production - and their associated different ranges of pressure to be measured (0-3000 psi and 0-15000 psi respectively), different dedicated sensor constructions are described. All transducers are designed to operate in the 0- 400°C temperature range. The requirement that the sensors meet the accuracy of 0.1% of the full measurement scale is discussed in details and is evaluated using analytical methods and simulation.