ABSTRACT Understanding the formation of carbonyl sulfide (OCS) in interstellar ices is key to constrain the sulfur chemistry in the interstellar medium (ISM), since it is the only ice S-bearing molecule securely detected thus far. Two general pathways for OCS formation have been proposed: sulfurization of CO (CO + S) and oxidation of CS (CS + O), but their relative contribution in interstellar ices remains unconstrained. We have evaluated the contribution of both pathways to OCS formation upon energetic processing in isotopically labelled CO$_2$:CS$_2$ and CO:CS$_2$ ice samples at 7$-$50 K. Our results indicated that formation of OCS through the CS + O pathway was more favourable than through the CO + S pathway, as previously suggested by theoretical calculations. In addition, its relative contribution increased at higher temperatures. Therefore, this pathway could play a role in the ice formation of OCS, especially in warm regions where CO is expected to be preferentially in the gas phase. At the same time, we have explored the chemistry of CS$_2$-bearing, CO$_2$-, CO-, and also H$_2$O-rich ices, that could be relevant to the sulfur interstellar chemistry. We observed formation of a variety of S-bearing products in addition to OCS, including SO$_2$, C$_3$S$_2$, and S$_2$. However, a significant fraction of sulfur was not detected at the end of the experiments, and could be locked in long, undetectable sulfur allotropes, one of the potential carriers of the missing sulfur in the dense ISM.