Tashdeed (or geminate) is a sign that goes above a consonant to show that that consonant has to be pronounced as if there were two of them. Listen to the examples:

For historical reasons, there are Persian words in which the letter “و” preceding the letter “ا” and following the letter “خ” is silent.

Worse than that, there are also words in which the letter “و” is pronounced like “o”, though it is not the last letter of a word. This includes most European loan words. Thus, we have two problems. First, when we encounter new words, there is a new potential pronunciation for the letter “و”. Second, when we want to guess the spelling of a word containing the sound “o”, a new possibility is that the letter “و” should be used instead of the short vowel sign which could be omitted.

There is no definite solution, but a helpful clue is that when “و” is read as “o”, almost certainly, it is either preceded by “خ” (this has historical reasons again) or it is in a European loan word.

A third trouble with the letter “و” is that it is sometimes pronounced as “ow” (sounding like the “ow” in the English word “grow”).