Olives are very slow growing and require little pruning. The crown of the tree will normally branch naturally so it is best left to its own devices. If you need to prune out any badly placed or frost damaged shoots then leave this until the spring.
As your tree is still quite young I would also suggest that you offer it some winter protection this year. If it is in a container then move it to a frost free greenhouse or cool conservatory. If it must stay outside then cover it with several layers of horticultural fleece during particularly cold periods and wrap some bubble wrap around the pot to prevent the roots from freezing.