United one-way awards are bookable on-line only, not by phone.
You can’t hold awards at united.com.
Hence, you cannot put one-way awards on hold.
If you don’t have enough miles in your account, you can buy miles.
But mileage purchases can take up to 48 hours to post. (Why can other airlines do this instantly??)
Normally not a big deal. While you normally need miles in your account with United to put an award on hold, you can put a United-only award on hold pending posting of purchased miles. (The requirement for miles in your account to hold an award is not one imposed by American and US Airways. And Continental only lets you put awards on hold when you don’t have enough miles in your account.)
But since you cannot put a one-way award on hold, and you have to wait up to 2 days for purchased miles to post, you’re forced to risk an award disappearing while you wait. And wait. And wait.