Searching the entire country is not available at the moment but it is possible to search a larger area than your city or province.
Because there are thousands of ads posted on OLX across the country, it's easier if you first type in what you're searching for. The results are organised by the nearest to you first.
The more you scroll down the results, the further the options get to you. So, it starts at anything matching your search within 1km of your location and then expands to 600km.