We don't know for certain yet. When Rand draws TP to balefire semi, a face (presumably Moridins) flashes in his mind suggesting that the link had activated. We also know from Demandred that the DO has limited TP to Moridin only. This has led to the speculation that Rand did not get the TP directly from the DO, but rather from Moridin through their balefire link.

Whether this is true, or whether it really was DO who gave Rand permission is something that we won't know for sure until the next or final book.