All those who have learned French at school know this sound. It is explained in detail here : the nasal o.
The graphem, om / on, is the only one who is the same in French and in Portuguese. We point out that this is the only nasale vowel who has the same phonetic value in Portuguese and in French. The other nasale vowels are similar, but not equal. In the following examples we have an r and de / te as well. Right now we don't care about them and will discuss about them later in 184.108.40.206.2 (r) and 220.127.116.11 (ti / te / di / de). (We don't care about the r and the de right now. The r is more complicated than in Spanish, because there are two of them and de is pronounced [dshi], -te is pronounced [tshi]. If someone cares: In pseudo phonetic transcription voiced sounds are often presented with d, because d is voiced, and t is often presented as t, because t is unvoiced.)
onde (where), monte (mountain)
bom (well), romper (break)
In opposite to French, where we don't have nasale diphthongs, to the monophtong exists a corresponding diphtong with two different graphems.
põe (he puts)
The diphtong shows up very often in the plural of nouns that finishes in ão in singular: (opinão (opinion) => opinões (opinions)).