6043 Background: Neoadjuvant treatment (NAT) has improved clinical outcomes in some patients with locally advanced oral squamous cell carcinoma (LAOSCC). Effective biomarkers assessing the treatment response is still lacking. Previous studies have posed the potential of circulating tumor DNA (ctDNA) in monitoring treatment response, but the underlying mechanisms behind non-shedding in tumors remain unclear. Methods: 29 LAOSCC patients received NAT followed by surgery were enrolled. Seven (24.1%), 9 (31%), and 13 (44.8%) patients received NAT of apatinib+camrelizumab, TPF chemoagents, and toripalimab+paclitaxel+cisplatin, respectively. Tumor tissues before NAT underwent whole exome sequencing and whole transcriptome resequencing. Blood samples at pre-NAT (T0), mid-NAT (T1), before surgery (T2) and after surgery (T3) were analyzed using a tumor-informed, personalized NGS assay. Multi-omics analysis, including ctDNA status, was performed with patients' clinical features and outcomes. Results: Among the 29 patients received NAT, the MPR rate was 55.2% (16/29), and the pCR rate was 27.6% (8/29). At baseline(T0), ctDNA was detectable in 25/29 (86.2%) patients, which decreased over time (T1: 77.8%; T2: 51.7%; T3: 6.9%). Persistent ctDNA at T2 indicates a higher rate of residual disease after NAT (100% non-pCR) compared to ctDNA-negative patients (42.8% non-pCR; p = 0.0047). All patients achieving pCR at T2 were ctDNA-negative (n=8, 100%) including 2 non-shedders. The PPV and NPV for predicting pCR by ctDNA clearance at T2 were 57.1% (8/14) and 100% (15/15), respectively. ARSF, CFAP47, or HAUS8 mutations at baseline were associated with significantly higher pCR rates (p<0.05). Four non-shedders with undetectable ctDNA at T0 were significantly enriched in patients with negative lymph nodes, lower clinical T stage, and high CPS score. The non-shedders had significantly lower tumor mutational burden (TMB), higher stromal score and cancer-associated fibroblast (CAF) (p<0.05). Spatial transcriptome analysis further supports that MHC-II pathway-related genes were significantly up-regulated in the non-shedders. The enhancing communication intensity between CAFs, endothelial cells and T cells might provide an anti-cancer immune microenvironment for non-shedders. Conclusions: Our results indicate that the clearance of ctDNA clearance at T2 was associated with an improved pCR rate. Furthermore, our findings shed light on the factors that prevent ctDNA release from oral squamous cell carcinoma.[Table: see text]