Nanoparticle assembly using oscillating magnetic fields at low frequencies is a scalable manufacturing method to tailor nanostructures within polymer matrices to provide effective multi-functional properties. Scalability and precise structuring of the nanoparticles in a fast and energy-efficient manner is a promising solution to the bulk fabrication of polymer nanocomposites (PNCs) to enhance mechanical, electrical and thermal properties. Thus, active assembly of nanoparticles using oscillating magnetic fields is studied here to evaluate its effectiveness on controllable and homogeneous assemblies of nanoparticles within a polymer matrix. In previous studies [1-3], parametrical evaluation of critical manufacturing parameters and their ranges on the assembly of iron oxide nanoparticles in low and high viscosity matrices, and the effectiveness of surface treatment on particle dispersion and suspension have been studied. This study presents continued experimental work on nanoparticle assembly using oscillating magnetic fields and the investigation of structure-property relationships of PNCs. In order to characterize the nanoparticle structures within the fabricated PNC samples, MicroCT inspections are performed. In addition, to investigate the structure-property relationships of the fabricated PNCs which consist of iron oxide nanoparticles in a thermoset matrix (Epon 862/Epikure W), electrical and thermal conductivities are measured in the axial (parallel to the applied magnetic field direction) and transverse (perpendicular to the applied magnetic field direction) directions. A preliminary study was conducted about the fabrication of magnetically-responsive elastomeric composites (magnetoelastomers); the assembly of iron oxide nanoparticles is studied in a high viscosity elastomeric matrix (polydimethylsiloxane, PDMS, 800 cP). After controlled hierarchical organization of the nanoparticles in the elastomeric matrix is achieved, triaxial assembly will be studied in order to advance knowledge regarding the three-dimensional magnetic structuring of nanoparticles for fabrication of magnetoelastomers with tailorable properties.