The interaction of localized surface plasmon resonances (LSPRs) with the photonic modes in an opal photonic crystal is reported. Gold nanocrescents are evaporated at glancing angles over both microsphere monolayers and artificial opals. Microsphere monolayers allow the three localised plasmonic resonances observed in the nanocrescent optical spectra to be identified. By changing the microsphere diameter, the opal photonic stop band and a selected plasmonic resonance can be spectrally overlapped causing a dramatic change of the properties of both excitations. The creation of new modes is observed, whose spectral position and dispersion properties are a combination of those localised in nanoparticle plasmons and in the photonic crystal.