I just changed mine out on a Cadet with OSP 700. This machine checks the voltage on the battery and generates an alarm when they get low. The other way it generates an alarm is when a certain time has expired, which requires a battery utility disk in order to reset the alarm. I believe yours will do the same.
Either way, replacing the batteries in advance is usually a waste of time unless you know that the machine is going into storage for quite a while IMHO.
That being said, if you open up the battery cassette (without unplugging it), you should have 3 toshiba ER17500V 3.6V lithium batteries. You can get them on e-bay or I've heard that batteries plus can "rebuild" the cassette for you. I opted for the first, but found that the wires supplied with the batteries had the correct connection, but were were too short to properly fit the Okuma cassette, so some additional effort was required on my part.
Once rebuilt, I just swapped them out with the machine under power to ensure no memory loss. (not the Okuma procedure... I know)
Used the utility to reset the alarm.
Best to make sure you are backed up and that the date is correct on your machine before starting this procedure.