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We present BOND, a Bayesian code to simultaneously derive oxygen and nitrogenabundances in giant H II regions. It compares observed emission lines to a gridof photoionization models without assuming any relation between O/H and N/O.Our grid spans a wide range in O/H, N/O and ionization parameter U, and coversdifferent starburst ages and nebular geometries. Varying starburst agesaccounts for variations in the ionizing radiation field hardness, which arisedue to the ageing of H II regions or the stochastic sampling of the initialmass function. All previous approaches assume a strict relation between theionizing field and metallicity. The other novelty is extracting information onthe nebular physics from semi-strong emission lines. While strong lines ratiosalone ([O III]/Hbeta, [O II]/Hbeta and [N II]/Hbeta) lead to multiple O/Hsolutions, the simultaneous use of [Ar III]/[Ne III] allows one to decidewhether an H II region is of high or low metallicity. Adding He I/Hbeta pinsdown the hardness of the radiation field. We apply our method to H II regionsand blue compact dwarf galaxies, and find that the resulting N/O vs O/Hrelation is as scattered as the one obtained from the temperature-based method.As in previous strong-line methods calibrated on photoionization models, theBOND O/H values are generally higher than temperature-based ones, which mightindicate the presence of temperature fluctuations or kappa distributions inreal nebulae, or a too soft ionizing radiation field in the models.

BOND: Bayesian Oxygen and Nitrogen abundance Determinations in giant H II regions using strong and semi-strong lines