Neil's floated free-kick appeared to be headed on by Thomas, who was marginally offside, and the ball was touched into the net by Wigan target McArthur to silence the Ibrox crowd.
The Ospreys thought they had scored the game's first try when the ball bounced into the in-goal area but Alun Wyn Jones was adjudged to have tackled Neil de Kock without the ball before he touched down.