. . . . . . . . S. D. O is . . . . . . . . officer of . . . . . . . . very high rank.

Use 'an' before words such as "hour" which sound like they start with a vowel even if the first letter is a consonant. Also use 'an' before letters and numbers which sound like they begin with a vowel, such as "F" or "8". 'An' will be used before 'S.D.O.' and 'officer'. 'A' will be used before 'very high rank'.