Yes, it’s possible to go one-way using points and the other way using money, or to use points for one return ticket and to pay for the other.
Your 325k AmEx points will get you 162,500 frequent flyer points. Probably your best value option is Asia Miles. Using Asia Miles points to fly Cathay Pacific, you’ll need 170,000 points for two one-way flights to Europe (or one return flight). It’ll cost 180,000 Asia Miles points to fly on other Oneworld airlines such as Qantas or Qatar. Other frequent flyer programmes require a lot more points (eg. KrisFlyer: 232000 points, Qantas: over 250,000 points). So Asia Miles is your best/only option with the points you’ve got. You can purchase points from Asia Miles if you don’t quite have enough.
The challenge is that Asia Miles availability out of Australia is generally pretty awful unless you book a full year in advance. Spend some time on the Cathay Pacific website, be willing to be flexible with dates, destinations, and (if necessary) departure cities, and you might get lucky.
If you can’t find a suitable reward flight, your options are to delay your trip until next year, save up more points so you could use something like KrisFlyer (whose availability is generally better), or pay for a flight to somewhere in Asia and use your points from there.
Let us know how you go!