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In optical OFDM (OOFDM) intensity-modulation and direct-detection (IMDD) single-mode fibre transmission systems using directly modulated DFB lasers (DMLs), wavelength-offset optical filtering considerably improves extinction ratios of OOFDM signals. However, optical filter-induced group delays bring about OOFDM signal spectrum distortions. Detailed investigations show that group delays associated with super Gaussian optical bandpass filters (OBPFs) significantly reduce both the dispersion tolerance and achievable optical power budget (OPB) of the system. To solve such a challenge, two simple approaches are proposed including combined optical filtering by using a ring resonator filter after a super Gaussian OBPF, and cascaded optical filtering by using multi-stage ring resonator filters. It is shown that, by optimizing ring resonator filter parameters, the proposed approaches can completely eliminate the filter group delay effect and enable 7 dB OPB improvements.