We are renting a duplex. The lease was up at the end of January, 2008 however, we have remained in the Unit due to not being able to find shelter elsewhere at the time. We now have found a home we are going to possibly take over the payments on and we are according to what I have read considered to be tenants at will right now. Do we still need to give a 30 days notice in Oklahoma to vacate the property when we are going to be basically purchasing this other home and are not under a lease agreement any longer?

Most likely 30 days but you would have to read if your lease says anything about when you go month to month or ask the landlord.

Although your lease has ended, you are now under a month to month rental, ( not really "at will" the terms of the lease as to what happens at the end of the lease will still be valid, if the lease states a 60 day notice, that will need to be given, if the lease says a 30 day notice that will be it.
Normally it will always be at least the 30 day notice if you are paying on a monthly basis.