The stability of oat-based drinks (OBD) is governed by its physicochemical properties. The measurement of these properties is often lengthy, laborious, and most often change the food matrix and introduce bias. Meanwhile, spectroscopic methods provide an in-situ rapid measurement of these properties. This study explored the use of several spectroscopic methods in combination with multivariate data analysis to describe the physicochemical stability of OBD. Samples of OBD with various properties and separation types were prepared from wholemeal oat and oat bran, with and without β-glucanase to simulate the presence and absence of the β-glucan network, and subjected to neutral and acidic pH. FTIR and NIR spectra gave the chemical fingerprint of the product. The first principal components from a PCA for FTIR and NIR both corresponded to the carbohydrates variation in the products (60.73% and 91.31% explained variance, respectively). In addition, in the overview PCA, the NIR spectra also correlates with the product's thickness in PC 1 (38.81% explained variance). LF-NMR T2 values of the fresh samples were able to describe the product's physical characteristics. T2,1 represented the bulk water which defined the product's thickness (T2,1 thick and thin products 197–505, and 704–1288 ms). In contrast, T2,2 represented the state of bound water, which could classify the product separation tendency. T2,2 values for stable, phase separation, creaming and complex formation are respectively 35–72, 55–78, 82–117, and 153–178 ms. Intrinsic fluorescence spectroscopy of the protein region showed that intact β-glucan network increased hydrophobic protein extraction. In the absence of β-glucan network, bran products has higher hydrophobic protein fraction than wholemeal products. Hydrophobic protein was sensitive to acidification and led to complex coacervation separation at pH 4.2 without β-glucan network.