The present work introduces a detailed study related to the synthesis of novel silicon oxycarbide based glasses and glass-ceramics as well as their in vitro bioactivity, i.e., their ability to induce surface mineralization of hydroxyapatite upon exposure to simulated body fluid (SBF). The focus of the work was to rationalize the correlations between the structural features of the prepared silicon oxycarbide based materials and their bioactivity. Thus, the effect of the glass network architecture, of secondary phases as well as of the specific surface area and porosity on the bioactivity of silicon oxycarbide was investigated. This was achieved upon modification of the silicon oxycarbide glass network with additional elements, i.e., B, Ca, Sr, which were shown to affect both the glass network architecture and the phase composition of the prepared silicon oxycarbides. Moreover, the specific surface area of the prepared materials was modulated upon adjusting their synthetic procedure, as shown exemplarily in the present work for Ca-modified silicon oxycarbide glasses. The incorporation of additional B, Ca and/or Sr elements into silicon oxycarbide modified its microstructure in three different ways: (i) by forming minor soluble secondary calcium silicate and/or strontium silicate phases as for Ca and Sr modification, (ii) by introducing Q3 units (silicon tetrahedra with one non-bridging-oxygen) into silicon oxycarbide amorphous network as for Ca modification and (iii) by reducing network carbon content as for B modification. The dissolution of crystalline silicate phases during bioactivity assessment of prepared silicon oxycarbide materials in SBF solution resulted in increased Si release, which was beneficial for apatite formation. On the other hand, both the formation of Q3 units and the decrease of carbon content in the glassy network decreased its network connectivity (NC). The less connected network architecture was responsible for the improved bioactivity of the investigated silicon oxycarbide materials. Furthermore, the slight network depolymerization effect upon Q3 formation was found to have a higher influence on the silicon oxycarbide bioactivity than network carbon content. Thus, slight network depolymerization in silicon oxycarbide based glasses is sufficient to achieve high bioactivity. The specific surface area and the porosity of silicon oxycarbide were modulated in a case study on sol-gel based Ca-modified silicon oxycarbide glasses. Thereafter, the introduction of Ca into the oxycarbide glass was shown to have significant effects on the structural features of the prepared xerogels as well as the resulting Ca-containing silicon oxycarbide glasses. Moderate content of Ca modifier was shown to generate mesoporosity in the xerogel and stabilize it against collapse; while higher content resulted in a significantly reduced surface area and porosity in the xerogel. Moreover, two main effects were observed in the resulting oxycarbide glasses: (i) moderate Ca content led to high surface area and amorphous glasses and (ii) high Ca content induced the formation of calcium silicate secondary phase. It was shown in this case study that high specific surface area, which was provided by relatively large fractions of mesoporosity, is highly beneficial for achieving high bioactivity.