BT Wholesale will also consolidate O2 UK's mobile and fixed networks using BT's 21st Century Network (21CN) platform.
The deal comes after O2 admitted network congestion problems in the London area following its launch of the iPhone. It said earlier that data traffic had risen 18-fold as a result of smartphones.
BT said the new backhaul deal will let O2 deliver a range of next-generation communications services and cut its capital expenditure.
O2 is known to have tested the 100Mbps Long Term Evolution (LTE) upgrade technology, but a spokesman declined to say if the firm would introduce it this year.
BT Wholesale already provided O2's core mobile network management support as well as a managed network service for O2's fixed and broadband customers.
Nigel Purdy, O2 UK's head of networks, telephonic, said, "As we move to an all-IP world and as data traffic volumes increase, the consolidation of our fixed and mobile core networks is a common sense approach."