This study was conducted to identify appropriate composting conditions for solids separated during the pretreatment processes of pig slurry treatment system, specifically focusing on primary solid-liquid separation using a screen and secondary solid-liquid separation using a decanter, through the evaluation of oxygen uptake rate (OUR). Seven treatment groups were used to compare primary screened solids (T1), secondary decanter-separated solids (T2), mixtures of primary and secondary solids without bulking agents (T3 and T5), and additional treatments incorporating sawdust as a bulking agent with different mixing ratios (T4, T6, and T7). OUR was evaluated to assess the biodegradability of different solid fractions. Oxygen uptake rates ranged from 7.1 to 14.2 mg O₂/g VS·d. The highest value observed in T2 (secondary decanter-separated solids). Lower oxygen uptake rates were observed in sawdust-amended treatments, indicating reduced microbial activity. This suggests that secondary decanter-separated solids contain more readily biodegradable organic matter and provide a favorable environment for composting. The reduced oxygen uptake rate in sawdust-amended treatments was likely attributable to excessive C/N ratios and the presence of lignin-rich recalcitrant material. Additionally, the increased porosity in sawdust mixtures may have contributed to a substrate dilution effect. Treatments combining both screened and secondary decanter-separated without bulking agents (T3) and (T5) exhibited intermediate OUR 10.3 and 11.5 mg O₂/g VS·d, respectively. Overall, these findings demonstrate that secondary decanter-separated solids (T2) represents the most suitable fraction for composting, whereas sawdust addition requires careful optimization to avoid nitrogen limitation.