How does too much wood taste?
You've tasted it: intense tannin astringency and bitterness, like sucking on a cheap tea bag.
I aged a beer in a French oak barrel for 3 months, today I popped open a bottle and my teeth feel like the enamel wants to come off. So not cool. I don’t get it, I tasted this same beer 2 weeks after bottling and I thought wow one of the best beers I’ve ever tasted (it was so smooth and velvet like almost like cream), and today 4 weeks after bottling and the enamel wants to come off my teeth. There is a bitterness that is like no other bitterness I have ever tasted, and it is so subdued almost like the oak is contributing its own IBU's.
Let the beer age in a cool, dark place for at least 6 more months. If you're lucky, the intense tannins (responsible for the astringent, resinous, tooth-coating sensation) will settle down, giving you vanilla, coconut and mildly oaky/woody notes. You did brew a beer which could stand up to extended aging, right?
If this isn't an option, your only choice is to rebrew the beer, age it in a glass/stainless container and then blend it with your current batch of beer to control the degree of oak character.
I'm not sure why you experienced the sudden flavor change, though. As a guess, there might be something in your beer that ate up some of the residual sugars and starches, making previously hidden tannins really stand out. Be on the lookout for possible wild yeast/bacterial infection. Sample a bottle of your beer every couple of weeks, if you're getting gushing bottles, thin body and off flavors, your beer is headed towards Lambic-ville.