Polyolefin compositions having improved properties comprise oligomeric poly(phenylene ether) as an organic filler. The composition comprises specific amounts of polyolefin, oligomeric poly(phenylene ether), and optionally a compatibilizer. The polyolefin can be a polyethylene homopolymer, an ethylene-containing copolymer, a polypropylene homopolymer, a polypropylene-containing copolymer, or a combination thereof. The oligomeric poly(phenylene ether) can be monofunctional or bifunctional. The compositions can have increased flexural modulus, heat deformation resistance, and flame resistance, compared to compositions without the oligomeric poly(phenylene ether). A compatibilizer can partially restore elongation when high levels of oligomeric poly(phenylene ether) are present.