Human antibodies, preferably recombinant human antibodies, both humanized and chimeric, which specifically bind to human OX40 are disclosed. Preferred antibodies have high affinity for OX40 receptor and activate the receptor in vitro and in vivo. The antibody can be a full-length antibody or an antigen-binding portion thereof. The antibodies, or antibody portions, are useful for modulating receptor activity, e.g., in a human subject suffering from a disorder in which OX40 activity is detrimental. Nucleic acids, vectors and host cells for expressing the recombinant human antibodies are provided, and methods of synthesizing the recombinant human antibodies, are also provided.