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Orthogonal frequency-division multiplexing (OFDM) is a multi-carrier modulation technology that transmits a high-speed data stream using multiple spectrally overlapped lower-speed subcarriers. Optical OFDM (O-OFDM) technology is a promising candidate for future high-speed optical transmission. Based on O-OFDM, a novel elastic optical network architecture with immense flexibility and scalability in spectrum allocation and data rate accommodation can be built to support diverse services and the rapid growth of Internet traffic. This architecture can provide various services directly at the optical layer in a spectrum-efficient way through bandwidth-elastic optical paths. However, carrying various data rate services using a single type of bandwidth-variable transponder might not be cost-efficient. Electrical traffic grooming is a traditional approach for sub-wavelength service accommodation in wavelength division multiplexing networks. However, it places additional electrical switching and optical-electrical-optical conversion requirements on the network, which may lead to higher cost and energy consumption. In contrast, grooming traffic optically is an attractive option for elastic optical networks. In this paper, we propose a novel optical grooming approach to aggregate and distribute traffic directly at the optical layer in OFDM-based elastic optical networks. We study routing and spectrum allocation algorithms of optical grooming to show the benefits of this approach. Our results demonstrate that significant transmitter and spectrum savings can be achieved by the optical grooming versus the non-grooming scenario, and a trade-off between optimizing the number of transmitters and optimizing spectrum usage should be considered during network planning.