Polyamide12 (PA12) has been used to fabricate flexible dentures for patients with prominent undercuts. It has a high moulding property, enabling it to take the shape of the canal without forming any voids, hence a suitable canal space restorative material. This study aims to investigate the chemicomechanical properties of PA12 composite reinforced with crystalline nanocelluloses (CNC) from oil palm fibre (OPF) and rice husk (RH) silica compared to commercial posts. Treated silica from RH and CNC from OPF were incorporated into the PA12 matrix to obtain the samples. Six 1.5 mm × 2 mm × 2.5 mm samples were prepared for seven groups. The flexure test, Fourier-transform infrared spectroscopy, and scanning electron microscopy (SEM) were employed during sample evaluations. A statistically notable variation in flexural strength among the sample groups [χ2 (df) = 38.65 (6)] was observed. The PA12 with CNC fillers recorded increased flexure strength than its unfilled counterpart and no significant difference with the commercial post. The SEM results confirmed the presence of both fillers in the composite. Smoother surfaces with fewer voids and gaps were observed with filler weight percentage (wt%) increments, suggesting enhanced interfacial bonding. Incorporating CNC into the PA12 matrix increased the flexural strength of the composite, whereas silica reduced it. Therefore, within the limitations of this study, new PA12 composites reinforced with CNC fillers from OPF biowastes could be a promising alternative for dental posts. However, additional research is needed to fully assess their viability.