There well and truly is no such word. I'm guessing it's a misspelling (more of a typo or transliteration accident, really) of овощ, which has nom. pl. óвощи and gen. pl. овощéй.
UPD. Oh! Dahl's dictionary actually does list обощ it as a southern dialectal form of овощ. The endings would be the same (standardly, anyway; the respective dialect might have been sufficiently Ukrainian-leaning to go for обощов or обощев), and with the lot of dialectal levelling-out in the 20th century there's a high chance the form is extinct by now, or else surviving in a handful of villages.