I know the historical OP are definitely used for replay, but are they also used for live trading?
You MUST have historical OP data in order to have the indicators available to make decisions on trading. There are two ways to get the data.
Download the data on Sunday, which will be current only up to that point.
Leave your computer on with Ninja, workspace, and charts open 24/7 (except for reboots once a day, and when market is closed) and your OP data will be constantly generated all the time.
Drawback: If you have your computer off from Monday through Thursday and want to start trading Thursday morning, the downloaded OP data is current only through Friday, nearly a week ago.
Historical OP data is not really used for replay. You download replay data and it is merely a market data stream. When you start the replay then the APEX toolkit prints the OP data in real time. You don’t need the downloaded OP data for that.
You can download the OP data, and replay data, load the right times, and you will be able to go back and review the market with the OP data.
Of course, everyone does his own thing differently. And there are probably more ways to do it.