The OF numbers refer to the NATO codes for each rank when judging corresponding ranks. OF = Officer OR = Other Ranks
Table for UK ranks is at this
linky
SO1,2,3 etc refer to grades of Staff Officers. An SO1 is a Staff Offficer post filled by a wg cdr/cdr/lt col (OF4); an SO2 is a Staff post filled by sqn ldr/lt cdr/maj (OF3); and SO3 post is filled by a junior officer (or sometimes a WO)).
Many people confuse the 2 systems thinking that SO1 refers to all wg cdrs. It doesn't, it only refers to those in staff appointments not those in command.
That said why an
RAF IBN refering to
RAF pilots needed to use the term OF3 escapes me - we only have one OF3 rank that is squadron leader.