In OctoPrint when receiving temperature data (M105) what is T0?
T is the selected tool, T0 is the first hotend tool. If you only have one hotend, T and T0 are exactly the same. 

Do note that G-codes are described on the G-code wiki page, for M105 you can find:


  The parameters mean the following:
   - T, T0, ..., Tn - extruder temperature. In a single extruder setup, only T will be reported. Some firmware variants will report no T0 in multi extruder setups - in that case T is to be considered the temperature of the first tool. Otherwise, T should be considered the temperature of the currently selected tool (which will be repeated in one of the Tn entries)
   - B - bed temperature
   - C - chamber temperature
   - @ - Hotend power (Prusa only)
   - B@ - Bed power (Prusa only)
   - P - PINDAv2 actual (Prusa MK2.5/s MK3/s only)
   - A - Ambient actual (Prusa MK3/s only)