This appears to be an As of Elagabalus, 218-222, I’m afraid the reverse is too far gone for me to figure out what it was. The obverse legend would have been something on the order of IMP CAES M AVR ANTONINVS PIVS AVG.
Luckily for us, that prominent “O” above the head is a strong indicator that it’s in the middle of “ANTONINVS” and since the earlier emperors who used that name in their titles, Antoninus Pius and Marcus Aurelius, were both bearded, that leaves us with only Caracalla and Elagabalus. The portrait doesn’t look at all like even a young Caracalla – who would have had a legend ending in CAES where this one ends AVG.
The portrait tends to resemble Severus Alexander – they were cousins, after all, but he wouldn’t have an “O” in his obverse legend.
Sorry I can’t tell you any more about the reverse – I just can’t make out any useful detail at all.

Weight: 8.16 g
Diameter: 24.92 mm