The pronunciation of words which begins 'con' and 'com'

A useful guide is that /o/ sound tends to occur when the syllable is stressed and the schwa /ə/ sound is usually when the syllable is unstressed.

For example,

cóntrast and contrást

cóntrary and contráry

cómbat and combátant

cómbine harvester and combíne

According to SPE -- The Sound Pattern of English (which I think is right about this matter), when the primary stress is on the second syllable, a heavy first syllable gets a secondary stress, but a light first syllable remains stressless, except for Latinate prefixes (like "con-"), which remain stressless even though they are heavy.

By "heavy", I mean that a syllable has a tense vowel or ends in a consonant. (The SPE account does not make reference to syllables, but that's what it boils down to.)

So, e.g., "computer" has no stress on the first syllable, even though it's heavy ("com-" ends in a consonant) because "con-" is a Latinate prefix. Compare "pentagonal", with secondary stress on "pen". Or, also, "campaign", with secondary stress on "cam".

In the SPE analysis, whether primary stress will fall on the first syllable of a word, like your example "contrast" (noun), is a separate matter.

A very reliable rule of thumb for British English and General American is this:

• If the syllable con is stressed it will take a full strong, vowel, and the first syllable will be: /kɒn/.

• If the syllable is not stressed we will find a schwa, /ə/, or no vowel at all in the first syllable: either /kn/ or /kən/.

However, we should bear in mind that some regional varieties of English will have a full vowel in the first syllable of many of these words regardless of stress. So in Yorkshire English, for example, many speakers will say /kɒnˈstɪtʃʊənt/.

This morpheme 'con' occurs a lot in English. That last sound in 'con', /n/, is very unstable both because it is an alveolar sound and also because it is nasal. It tends to change according to the following consonant. We find this prefix with different final consonants in words like: collect, commemorate, correlate. These types of word also show the same variation in their first syllables depending on the stress:

• colleague: /ˈkɒli:g/
• collection: /kəˈlekʃn/
• compensate: /'kɒmpənseɪt/
• computer: /km'pju:tə/
• correlate: /ˈkɒrəleɪt/
• corroborate: /kə'rɒbəreɪt/

Both words collection and corroborate could be said with no schwa at all, and just a syllabic consonant - but the transcription for this is tricky in terms of syllabification, so I have taken the easy route and used a schwa. Similarly, computer could equally be said with a schwa instead of a syllabic /m/.

[Transcription note: in line with the Original Poster's question I have used Southern Standard British English transcriptions].