Need help calculating current and understanding Vos in an op amp
Va is a separate node, which has a considerably different voltage than Vo. The input bias current of the op-amp is assumed to be zero, so the current i1 is common to the two resistors, and can be calculated as Vos/1M\$\Omega\$ or 4nA. 
The Vos is the 4mV offset voltage represented as a voltage source on the non-inverting input of an otherwise ideal op-amp. It's internal to the op-amp or you can draw it as external to an ideal op-amp.