I have an oil burner and want to add and air conditioner unit,do I need a second 24v thermostat for the cooling circuit

You're adding an AC unit to the existing system - i.e, you have a forced hot air system, correct? Doing this work will require substantial revamping of the furnace - have you considered going w/ a complete upgrade, i.e. replacing the old furnace w/ a complete new heating and AC unit?

Well, to answer your question - yes, a single thermostat can control both heating and cooling, but they typically require a 3-wire circuit. So if the thermostats in your home have a 3-wire cable coming to them you will simply need to replace the heating-only thermostat with a new heating/cooling one. If you only have 2 wires (which is what I bet you have), then you will need to run a new cable from the heater/AC unit to the thermostat.

Depending how old your system is, for the controls, you may need to do no more than run a pair of wires from the furnace out to the new A/C. You may need to replace thermostat and run a 4-5 wire cable. The big question is does the blower have the capacity for the higher air flow an A/C requires. At least part of the A/C installation must be done by a licensed technician. It will include adding an ''A'' coil to the furnace. Ask several contractors to give an estimate. They will know what to look for as far as your current system.