Torrefaction is a key process for biomass energy densification ahead of its utilisation in pyrolysis, gasification or combustion, and therefore the changes it originates have been the subject of multiple studies. Most of them employ low heating rates, in the range of thermogravimetric analysers (TGA), typically below 1 °C s−1 or high rates (100–1000 °C s−1) in reactors where tars undergo considerable secondary reactions following release from a particle and the temperature history of the sample is not well defined, such as entrained flow or fluidised beds. This study aims to analyse the behaviour of torrefied lignocellulosic biomass (oak) under fast pyrolysis in with very controlled temperature history and absence of secondary reactions between evolving volatiles and chars in a wire mesh reactor (WMR), which allows determination of primary products. To this end, oak was firstly torrefied in a grey-King reactor to provide samples from mild to severe torrefaction (210–300 °C). The raw and torrefied oaks were pyrolyzed/gasified, at 1000 °C s−1 and 850 °C in the WMR using He and CO2 atmospheres. All samples showed high volatile production, in excess of 80 wt.%, but with a clear drop with increasing torrefaction temperature. The char yields from the WMR in CO2 were lower than in He, reflecting a degree of char gasification. However, char deactivation at the higher torrefaction temperatures caused a sharp decrease in the extent of gasification from 32 wt.% for Raw Oak to 6 wt.% for oak torrefied at 300 °C (Oak-300). This is a consequence of the predominance of chemical effects such as enrichment of carbon, which increased from 46.5 wt.% for Raw Oak to 54.3 wt.% in Oak-300 and depletion of oxygen, which led to greater aromaticity and structural order in the char with torrefaction temperature, over physical changes observed by scanning electron microscopy (SEM) revealing increasing porosity.