Examples of the stamps issued by Western Powers in the Ottoman Empire
image courtesy of Jean Mirzan
image courtesy of Jean Mirzan
image courtesy of Jean Mirzan
The post offices used “home-made” stamps, with the word “Levant” and local currency overprinted on it. The exchange rate was as follows:
1863 onwards, 40 para = 1 piastre.
1929 onwards, 40 para = 1 kuruş.
100 kuruş = 1 lira

Historical review of Foreign Post Office history in the Ottoman Empire by Jean Mirzan, 2010:
The 1st post office to open was an Austrian post office at Constantinople after the treaty of Passarowitz in 1721, just for diplomatic exchange between Vienna and Constantinople. Similar offices were established by the Russians for communication between St Petersburg and the Ottoman capital.
From 1729 Austrian and Russian post offices started accepting mail from traders.
In 1783 a special agreement was reached with Russia in order to maintain a reliable service.
Similar agreements were signed with Austria in 1784, with France in 1812, Great Britain in 1832, Greece in 1834, Germany in 1870 and Italy in 1908.
Within the Ottoman Empire the estimated total number of post offices opened by respective nations were as follows: 37 Austrian, 24 French, 20 Russiam, 8 German, 9 Italian and 5 British.
Most of post offices were closed during World War I but reopened in 1919. With the treaty of Lausanne all foreign post offices were closed for good.
Regarding Polish and Rumanian post offices I could not find any reliable information.
All these informations are part of the exhibition I’m preparing, scheduled for the 4th to the 10th November, 2010, in Izmir.

Click here for further information on the history of the British Post-Offices in the Levant.