BYZANTINE, Andronicus III 1328-1341 Thessalonica

Obv: Emperor and St. Demetrius holding a long cross between them, with a star overhead.
Rev: Winged patriarchal cross (indistinct)
17 mm, .47 g.
Sear 2483
This coin is overstruck on a debased non-Byzantine denier of some sort, which I'm trying to identify (if you can identify it, please let me know!). On the obverse, directly over the star is an "O." To the left of the O is an indistinct letter, and to the left of that is a cross. The reverse looks like a heraldic shield with some lettering around it, indistinct except for a dot followed by an "A" at the top.