The 24 rows on the Orchestra Level are named alphabetically. The letters "I" and "O" are left out because they bare too much resemblance to numerals and could cause confusion on tickets!
According to playbill.com
• Almost all Broadway theatres have no Row I, but there are some holdouts. In both the Broadway and the Jacobs, there is a Row I in the orchestra section, but Row I is skipped in the mezzanine. Off-Broadway theatres also tend to skip Row I.
• Some theatres have omitted other rows as well. The Jacobs skips Row Q in the orchestra. The New Amsterdam skips Row I and Row O, as does the Walter Kerr and the Marquis. The Neil Simon skips Row I and Row Q. (We're going to ignore the quirks that occur at the far ends of the alphabet in many theatres, including double-lettered rows such as AA or ZZ).
• It's pretty easy to see what's going on here — O and Q can get confused. A spokesperson for Mary Poppins, which is at the New Amsterdam, said that the theatre is actually concerned with confusion with the numeral zero, and specifically with the instances in which ushers and audience members are trying to look at the tickets and find seats in the dark during a show. The New Victory, a children's theatre on 42nd Street, also skips Row O (in addition to Row I) because of its confusion with zero, according to a spokesperson.
• In London, not surprisingly, things get even weirder. The National Theatre's Olivier Theatre and Lyttelton Theatre skip Rows I, Q and N — presumably because N looks and sounds like M. The Theatre Royal, Drury Lane, which currently houses The Lord of the Rings, skips Rows I, Q and V. In the Palace Theatre, which houses Monty Python's Spamalot, the stalls (a.k.a. orchestra) include Row I but skip Rows Q and U, the dress circle (mezzanine) includes Row I, but the balcony skips Row I. Presumably, the similarity between U and V is also an issue.
Of related interest: Sports stadiums skip rows, too. For instance, Yankees Stadium skips Rows I, O and Q.
Have you ever noticed that the rows I and O are missing in Thrivent Hall?