Purpose of this oscillator (74HCT14-based) on RS-232 to RS-485 converter?

Two members claim opposite things, one in a comment and one in an answer. User jms has got it right. The oscillator generates +Vcc if -Vcc happens to be available as a signal in the connected RS232 port. The diodes connect possibly available -Vcc to the oscillator IC. The oscillator charges CT3 from -Vcc and the charge is pushed to maintain +Vcc. That pumping is really needed, if -Vcc happens to be the only available voltage via the RS232 port.

If the port happens to output (through a diode) +Vcc, it's used directly and the voltage minus Vcc is not absolutely needed, altough it helps the stability of the +Vcc voltage.

The design assumes that at least one of voltages -Vcc and +Vcc is available from the RS232 port. RS485 circuit needs only the positive one.