Oiapoque is a border town in Amapá in Brazil, right across the river from French Guyana.
Oiapoque lives in the Brazilian popular culture as the end of their long coastline, starting in Chuí.
There are lots of restaurants and stalls on the riverside, where the boats from Saint Georges arrive.
Saint-Georges in French Guiana is a 15-30 min boat ride away and cost R$15 or €5 in February 2017 . Be aware, though, that the customs office in Saint Georges is only open before noon, on 2 days in the afternoon and not at all on Sundays. If you want to travel through French Guiana, make sure you arrive before 1pm.
As the river bridge eventually opens for traffic you can cross the border even by foot.
If you come from French Guiana and want to go on to Macapa you can either take a regular bus or hitch a ride with one of the 4WDs that leave from the port (if you arrive by boat you will inevitably be asked whether you want that). The price for the latter is R$150, but it is at minimum a 7h ride and 4WDs at least have a better chance to get through otherwise impassible sections on the stretch of road that is not paved and turns into a mud pool after rain.