What does O stand for in V_CBO and V_CEO?
The "O" in \$V_{CBO}\$ is short for "Open", not attached to anything.

\$V_{CBO}\$ will be the voltage at collector with base as reference (0V) with emitter not attached. For NPN transistors this is usually a positive number in tens of volts.

Same goes for \$V_{CEO}\$ where base is not connected.

I think you get the picture with \$V_{EBO}\$, though usually approximately 6V.

In case of a PNP transistor, same parameters are used but the values for it will be negative.