Question-and-Answer Resource for the Building Energy Modeling Community
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Your problem is that the zone equipment is autosized, and during the sizing it just doesn't "see" your added load from dumping unconditioned air into your zones. It assumes the central air handling unit is providing "neutral" temperature air. In other words, your zone equipment is sized to meet only the thermal load (internal gains and envelope losses).
So you end up with a system that is undersized to meet thermal loads + ventilation loads, and therefore you get many unmet hours.
If you want the sizing to work correctly for your case with a Dedicated Outdoor Air System (DOAS, or OAU as you call it), I think you'll have to change your infiltration object to have an increased value for design days only that equates to your typical infiltration + the Outdoor Air CFM from the DOAS unit. This will be painful if you have a lot of zones though.
Source: EnergyPlus Support Group thread: Why autosize doesn't consider System Outdoor Loads
