Greeks drive on the right side of the road, like all continental Europeans. However, greek drivers are notorious for bending the law, when it comes to driving. So, do not assume that, because something is illegal, Greeks will abstain from it.
When driving down a one-way road, you may often encounter motorbikes, or even cars, driving in the wrong direction, and coming towards you in an attempt to shorten their overall distance traveled.
Slow down a little bit, and keep driving ahead while paying attention to leave some space on your left side, in order to allow the on-coming motorbike driver through.
If the on-coming guy is driving a car, and it’s not easy for you to accommodate the situation by reversing, or making way for him to drive through, then stand your ground, and insist that he either finds an empty spot, to make way for you to drive through, or that he drives in reverse all the way back. Do not yell at the other driver, stay calm and firm.