Can a single bjt PNP be an or gate?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A tidy version of the OP's 'OR' gate.

It might work in certain circumstances. The problem is that the base current affects the emitter-collector resistance.


If A is high then an emitter-base current will flow. 
This will turn on Q1 and provide a low-resistance path between A and B.
What happens next will depend on the output impedances of A and B. If they are the same then Q1 will tend to pull them together towards mid-supply voltage.


It's not going to work as you planned.







simulate this circuit

Figure 2. Simulation circuit.

Running a simulation using the CircuitLab tool results in the following readings for R1 = R2 at 1k and 100 Ω:

     1k         100 Ω
A    1.927 V    2.469 V
B    1.909 V    2.365 V
Q    1.163 V    1.663 V


Note that at lower source resistance the effect of R3 is less and A and B settle down close to mid-supply. Q is 0.7 V below the emitter voltage as would be expected.