Obvenir is a french third group verb. So, this verb is irregular and do not follow a regular conjugation pattern like first or second group verbs. Follow this link to see all the endings of the conjugation of most of the third group verbs :
conjugation rules and endings for the second group verbs.
Verbs derived from
tenir and
venir form a major group of verbs who gains a
d in the future and conditional. These verbs are the only verb to have a nasal vowel in the past historic and imperfect subjunctive.