Omer (עוֹמֶר) really is pronounced /ˈoʊ̯mɚ/ or /ˈoʊ̯mɛɹ/ in Ashkenazi English. I would pronounce Lag b'Omer for example as /lɑɡ bʌˈoʊ̯mɚ/. Some communities would even say /ˈɔɪ̯mɚ/. But pronouncing it with stress on the last syllable is ungrammatical. Even in the original Hebrew, /ˈʔomɛʁ/ (Modern) or /ˈʕoːmɛʀ̟/ (Tiberian) is stressed on the penultimate syllable, not the final syllable.