Koscielny sealed an £8.5million move yesterday and long-term target Marouane Chamakh was signed before the World Cup started.
Wenger has been out in South Africa working as a pundit for French television but has had one eye on strengthening his squad further.
He said: "Koscielny can be an outstanding addition to our team, but it is not over, we are still open to buy.
"We are still on the market for another defensive player because we have the need for one more."
Bolton centre-back Gary Cahill as well as German defender Serdar Tasci are being linked with a switch to the Emirates, while rumours persist surrounding free agent Joe Cole, Manchester City winger Shaun Wright-Phillips and Brazil midfielder Felipe Melo.