The [OII]3727 emission line is frequently used as an indicator of the star formation rate (SFR) despite its complex dependence on metallicity and excitation conditions. We have analysed the properties of the [OII] and Halpha emission lines for a complete sample of local Halpha-selected galaxies, the Universidad Complutense de Madrid (UCM) survey. We find a large scatter in the [OII]/Halpha line ratios, although the scatter in the extinction-corrected [OII]^0/Halpha^0 ratio is considerably smaller. We also find that the [OII]/Halpha ratios are reasonably well correlated with the absolute B- and K-band magnitudes and with EW([OII]). However, the extinction-corrected [OII]^0/Halpha^0 ratio is largely independent of these quantities, indicating that extinction is the main driver of the correlations. These correlations allow us to statistically predict--with varying degrees of accuracy--the observed and extinction-corrected Halpha fluxes from the observed [OII] flux using the information contained in EW([OII]) and/or the absolute magnitudes, but extreme caution is needed to make sure that the sample selection effects are correctly taken into account.