Neuter plural.
The word is found only here and in the scholia on Aristophanes, Clouds 187ff., where the characters discuss people groveling in the earth for bulbs to eat. The commentary says that they were looking for bolboi, which it defines as onion-shaped and very similar to small onions. (If the Greeks did not know truffles, it probably refers to shallots.) It continues that some erroneously called them οἶδνα , either because they cause the earth to swell ‘οἰδαίνειν (omicroniota 27, omicroniota 28) as the plant emerges (citing Tzetzes, but not found there) or because they are wet (here rejecting an implausible etymology from ὕω ‘be wet’). The word is not found in the sense attributed to it in this entry.