If you use oak cubes, you'll have better control over the oak, the chips usually release all of their oak within a few days making it easier to over oak, oak cubes or oak beans are usually recommended at a rate of 1-3 oz per 5-6 gallon carboy and take 6 months or longer to release their full oak into the wine. 2.5 -3 ounces is considered to be equivalent to “new barrel” extraction rates.
If you're not barrel aging, it is best to add the oak prior to fermentation, when adding oak in the primary, the active yeast metabolizes and buffers the oak. much like a barrel ferment. (Thanks to Zac brown for that info)
I hope that this helps.