I would always use one, Ali, on account of Him being omnipotent, creator of the universe etc - it seems the least we can do is give the chap a capital letter!
Seriously, though, the oath originated within the framework of the Christian religion, so you're distinguishing Him from the general noun 'god'. I suppose some people would argue that in our secular society we've moved on, but I wouldn't be one of them.
<Added>It might be one of those words in transition, like all right/alright. Traditionalists like me would still type 'all right' as two words, but 'alright' is becoming so familiar it's accepted.