Polymer-derived ceramics (PDCs) represent a relatively new class of materials due to a series of advantages, such as chemical, thermal, and oxidation stability, over classical ceramics. In this study, iron-, cobalt-, and nickel-modified silicon oxycarbides (SiOC) are synthesized at different temperatures. The order of catalytic activity is iron > cobalt > nickel, in agreement with the activation energy calculation. The different phase compositions of the Fe-, Ni-, or Co-activated SiOC materials are rationalized by in-depth understanding of the phase composition and crystallization of the SiOC systems. Electrically conductive C-SiOC-SiC and C-SiC-TiC nanocomposites with a dense and homogenous structure can be obtained upon flash sintering under electric field, Joule heating and electromigration. Through proper precursor and additive selection, as well as thermodynamic and kinetic understanding, in-situ formation of C-SiOC-SiC and C-SiC-TiC nanocomposites is realized via phase separation and carbothermal reaction. High oxidation resistance and thus thermal stability are achieved.