Personally, I always use dated tickets. There is no cost to change the date, time, flight number, carrier etc on a OneWorld Explorer ticket so long as the routing remains the same.

If you have a specific flight included in the booking, and the carrier changes their schedule or even drops the routing, you are covered and the carrier will need to find an alternate way of getting you to your destination for no additional cost. If you have an open-dated ticket and later find the destination or routing is no longer served by OneWorld, it is your problem and not the carrier's problem. You will need to have the ticket re-issued and incur the costs associated.

So my recommendation is to estimate when you will want to travel and include specific dates/flights in the booking. You can change as much as you like later for no cost. And once you have flown the first international segment you can make these changes directly with the airline and do not need to work through the original travel agent.