No, you will be able to play D:OS single player.

From a comment in Divinity – Original Sin revealed !
Quote
The idea is that the inclusion of the cooperative mode actually increases the quality of your single player experience. How that's going to work exactly I'll reserve for a future post, but rest assured that we're focussed on making single player as cool as we can.

And the topic Heated Debate @ RPGWatch
Originally Posted by Macbeth
I can only corroborate what Swen said before: the single player experience is just as important to us as the multiplayer one.

Originally Posted by Lar_q
Yeah, I've seen that which means we didn't communicate very well, but t think the debate is going to move away as we show more and more stuff. The cooperative mechanics translate very well to party mechanics in single player, and the system is much more complex than might be surmised from the little details we've shown so far. Rather than type essays about it, I think we'll address this in a future video under the motto show, don't tell smile