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We have conducted the first systematic search for interday variability in alarge sample of extragalactic radio sources at 15 GHz. From the sample of 1158radio-selected blazars monitored over a $\sim$10 year span by the Owens ValleyRadio Observatory 40-m telescope, we identified 20 sources exhibitingsignificant flux density variations on 4-day timescales. The sky distributionof the variable sources is strongly dependent on the line-of-sight GalacticH$\alpha$ intensities from the Wisconsin H$\alpha$ Mapper Survey, demonstratingthe contribution of interstellar scintillation (ISS) to their interdayvariability. 21% of sources observed through sight-lines with H$\alpha$intensities larger than 10 rayleighs exhibit significant ISS persistent overthe $\sim$10 year period. The fraction of scintillators is potentially largerwhen considering less significant variables missed by our selection criteria,due to ISS intermittency. This study demonstrates that ISS is still importantat 15 GHz, particularly through strongly scattered sight-lines of the Galaxy.Of the 20 most significant variables, 11 are observed through theOrion-Eridanus superbubble, photoionized by hot stars of the Orion OB1association. The high-energy neutrino source TXS0506$+$056 is observed throughthis region, so ISS must be considered in any interpretation of its short-termradio variability. J0616$-$1041 appears to exhibit large $\sim$20% interdayflux density variations, comparable in magnitude to that of the very rare classof extreme, intrahour scintillators that includes PKS0405$-$385, J1819$+$3845and PKS1257$-$326; this needs to be confirmed by higher cadence follow-upobservations.

The presence of interstellar scintillation in the 15 GHz interday variability of 1158 OVRO-monitored blazars