You're basically SOL when it comes to filtering, but you can do a few things to make finding whatever OA form(s) are being referenced/overridden.
A) Tick the plugin's ESM flag in its file header, then reload. From there, look at the plugin's ONAM list for any forms starting with OA's load index and Ctrl+Click them from within the ONAM list (now in the file header), one by one, removing and and all OA CELL, WRLD, or NAVM form(s). After removing any given record like that, you can click the '<-' up top to go back to your spot in the ONAM list. They'll be listed in order with Fallout3.ESM overrides first.
B
) If after doing the above, "Clean Masters" does not remove OA from the plugin's masterlist, try deleting [NAVI:00014B92] and try "Clean Masters" again. If you delete NAVI, untick the ESM flag afterward, load, then save in the GECK and NAVI will be rebuilt. 'Cleaning' the mod first will remove any 'dirty' records, making it easier to isolate what needs removing.
C) From there, the only remaining possibility is that one of the other record groups has an override or reference to an OA form, so open 'em one by one and sort by FormID to make it easy to spot anything in OA's range, then remove it/them when you find them.