Hello
If you meant switching loss of pnp transistor, it is the loss during the transition of the state of transistor. Think your open collector output is high. That means transistor is off (eg collector=5V) and there is no current flow (except small leakage current). So V*I is nearly zeor. Similarly when transistor is activated, there is a current flow on the transistor but output voltage is nearly zero. Thus again V*I is very small. However during transition time (from 5 to 0 or vice versa), but voltage and current is non-neglgible. Thus switching losses dependent on frequency happens.
I guess it helps.
Regards