Symbol representation for IC
The default reference designators used on the schematic editor on this site can be seen below. There's an obvious preference for a single letter prefix.



simulate this circuit &ndash; Schematic created using CircuitLab
However, there's no standard. I tend to use IC for ICs, and TR for transistors, whether BJT or FET. This is the only place I've seen OA used for op-amps. I have seen Q used quite often for transistors, and X and U for ICs. T tends to mean transformer rather than transistor, though with the preponderance of one over the other in circuits, I'm surprised that T isn't seen more often to mean transistor.