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Actually on SQ, "O" is first class, "I" is business class and "X" is economy class.
Unfortunately the various availability tools (eg Expert Flyer) dont show SQ award classes. So the only way to find out availability is ringing up your FFP, or if a Kris Flyer member with sufficient miles checking online the availability (start of process for booking award online).
Note if looking for F awards, SQ typically only releases 1 per flight (maybe a second closer to departure) and only a few business award seats per flight. So I suggest get in as early as possible.