O & O27 track can be put together
but
the O27 will have to be shimmed to match the O track height.
Someone pointed out in an earlier post that the pins are of different diameter,
so that has to be dealt with.

I have old track, so mixing & adjusting connections are part of the Zen of toy trains.

Happy Rails!
Dave

If your train will operate on the O27 curves without binding and you can fit the track together, I'd give it a try.  Just remember to keep the speed down or cover the floor with pillows to soften the impact when you launch your locomotive into the ether, high balling into the curve.
 

There are special track pins to connect '027' with '0' gauge track. The Train Tender carries part #2948-10 http://www.ttender.com/partslist.html
As has been stated, due to the different heights of the tracks, the '027' track will have to be shimmed to match the height of the '0' track.

Larry