Why are Oscar, India, and X-Ray (O, I, and X) not used as taxiway identifiers?
Because those symbols are easily confused with the number 0, the number 1 and the symbol for a closed runway/taxiway (✕) respectively.

(1) Numbers  by  themselves,  and  the  letters  &quot;I&quot;  and  &quot;O&quot;  must  not  be  used  because  they could be mistaken for a runway number.
(2) The letter &quot;X&quot; must not be used because a sign with an &quot;X&quot; could be misconstrued as indicating a closed taxiway or runway.

Source: FAA Engineering Brief No. 89 on Taxiway Nomenclature Convention (via SKYbrary.aero).
Most aerodromes do refrain from using these identifiers, but there are also examples of them being used. For example, Copenhagen (EKCH) has a taxiway I(ndia).