Ơ is one of the 12 Vietnamese language vowels. It is pronounced [ɤ] (an unrounded [o]).
As with most special Vietnamese letters, this letter is not well-supported by fonts and is often typed as either o+ or o*. The VIQR standard is o+.
On the Windows default Vietnamese keyboard Ơ can be found on where the ] key is on a US English keyboard layout.
Because Vietnamese is a tonal language this letter may optionally have any one of the five tonal symbols above or below it.
References
Ơ Wikipedia(Text) CC BY-SA