Legend! I have now found the details thanks! I did search for his testimonial and found this
1986-08-05: Arsenal 0-2 Celtic, David O'Leary Testimonial
Match Pictures | Matches: 1986 - 1987 | 1986-87 Pictures | Friendlies - Away
Pierce O'Leary captains Celtic on the night in his brother's testimonial. David also captains Arsenal.
8,000 Celtic fans pack in to the away end at Highbury.
Game kicked off 15 minutes late to allow travelling Celtic fans to get in.
Alan McInally was taken off only to be sent back on later to score as Celtic only had 3 subs and all were used up.
This was George Graham's first game at Highbury as Arsenal manager.
However a further search finds this! Thanks!!
17/05/1993 David O’Leary Farewell Manchester United 4-4 22,117