This depends on the way you select assets when setting up your ABO strategy.
If you select assets using a filter, the same filter will apply to new ad sets in the future, which means the ABO will work on them if they satisfy the conditions.
However, if you choose to manually select ad sets, the strategy will not apply to any new campaigns. It may apply to new ad sets created in selected campaigns if you set it up to include all future ad sets within the campaign. Make sure to click twice until you see the ‘A’ for this option.
Note: Your ad set needs to spend enough to be able to drive results before the ABO can optimize it. The definition of enough would depend on the cost per event selected as an optimization event before the automation kicks in. If not enough budget is spent, the ad set is considered immature and not ready to be optimized.