What does the saying were Oscar mike mean in call of duty modern warfare 2

On (the) Move

It's derived from the phonetic alphabet. Oscar for 'O' and Mike for the letter 'M'
It's made and used for ROP or Radio Operating Procedure to avoid confusion over the net. You'll also note that the term 'niner' is from the same phonetic alphabet.
Additionally you might remember that the COD website used to be 'CharlieOscarDelta' dot com at one point. Which spells COD or Call Of Duty.

NATO Phonetic Alphabet Letterphonetic letterAAlphaBBravoCCharlieDDeltaEEchoFFoxtrotGGolfHHotelIIndiaJJulietKKiloLLimaMMikeNNovemberOOscarPPapaQQuebecRRomeoSSierraTTangoUUniformVVictorWWhiskeyXX-rayYYankeeZZulu


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