Specific supramolecular interactions in polysaccharides change their solubility rendering diverse interfacial properties in oil-in-water (O/W) emulsions. We studied the effect of solubility of softwood hemicelluloses, spruce galactoglucomannans (GGMs) isolated by pressurized hot-water extraction methods or recovered as side streams in a thermomechanical pulp (TMP) mill on interfacial structures and stability of O/W emulsions. Detailed characterization of GGMs revealed a soluble (molecularly dispersed) fraction of molar mass 8.9 × 103–2.1 × 104 g mol−1 and sub-micron-sized insoluble fractions that were either loose assemblies or fractal type supramolecular aggregates and agglomerates. Based on the relative share of these fractions and nanostructural complexity, GGMs were classified into soluble, semi-soluble, and insoluble. GGMs extracted via pressurized hot-water extraction methods were soluble to semi-soluble compared to GGMs recovered from the TMP process. Semi-soluble GGMs exhibited efficient stability of dispersed oil droplets in emulsions followed by insoluble and soluble GGMs. With an increasing share of insoluble fractions and their structural complexity, emulsion’s interfacial morphology changed from smooth to diffused type. Comparing GGMs with soluble small-molecule surfactant Tween 20 and insoluble alkali extracted beechwood glucuronoxylans, our findings suggest that surface activity of adsorbing soluble or insoluble fraction dominates the interfacial morphology. Under the condition of sufficient interfacial coverage, insoluble fractions complement the emulsion stability with a filling effect in the continuous phase of emulsions. The findings improve our understanding of bio-based polysaccharides’ solubility, their emulsion stability mechanisms, and strategies to tailor via biorefining approaches.