From the OKC blog Welcome to Loud City in their recap..
The other big surprise/disappointment was how poorly the Thunder defense performed against a mediocre Celtics offense. To be sure, sometimes teams get hot, and Boston certainly made a good share of baskets on the night, hitting 48.8% from the floor and 9-17 from 3-point range. However, on this night the Thunder seemed disinterested in playing the kind of intense defense that we had seen in the past few games, and the Celtics used the extra space to their advantage. Boston only averages 97 points per outing, yet the Thunder gave up 87 points in the final 3 quarters. OKC's poor defense was clearly a determining factor in the game's final outcome.
The Celtics did get hot (in particular from outside). But I suppose you can give an assist to the OKC defense, or seemingly lack thereof as well.