I'd suggest not specifying a target for the Oscar, unless scripts are being used. Under wartime ROE, the Oscar should engage hostile contacts without any specific orders to do so.
Another possibility is that the problem relates not to detection, but to classification. (Although for sonar contacts, both occur simultaneously---but, the ranges are fudged to account for this). Try putting the Oscar at PD and having a playable on the link so you can verify that the Oscar knows the contact is hostile. If you're playing stock DW, the Oscar has substantially less sonar capability than an Akula, so it might not engage at the same range an Akula would.
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