ExpertFlyer listed the delay as both "Operational" and due to a late arriving aircraft. From my limited understanding of BA delay codes the ZO operational code is different to a flight/cabin crew issue so perhaps the original plane went tech?
A friend's flight from Palma - LHR on BA 451 this evening followed exactly the same pattern whereby the original plane was replaced with another and showed the same delay codes.
Results from https://www.ExpertFlyer.com
Code:
Flight Status Search:
Departing on 08/03/17
Flying BA flight 782
Flight Status Depart Location Depart Date Reliability Arrive Location Arrive Date
LHR Scheduled: 08/03/17 5:00 PM ARN Scheduled: 08/03/17 8:35 PM
BA 782 Landed Term: 5 Estimated: 08/03/17 6:39 PM 77% / 25m Term: 2 Estimated: 08/03/17 9:53 PM
Gate: A15 Actual: 08/03/17 6:39 PM Gate: 86 Actual: 08/03/17 9:53 PM
Comments:
"DOBA782/03AUG
* OPERATIONAL FLIGHT INFO * BA 782 0 TH 03AUG17
CITY INFO HOUR (LOCAL)
LHR ESTIMATED TIME OF DEPARTURE 1832
LEFT THE GATE 1839
TOOK OFF 1902
DELAY ZY
DELAY ZO
ESTIMATED TIME OF ARRIVAL 2159 ARN
ARN AIRCRAFT LANDED 2149
ARRIVED 2153
*1A PLANNED FLIGHT INFO* BA 782 0 TH 03AUG17
APT ARR DY DEP DY CLASS/MEAL EQP GRND EFT TTL
LHR 1700 TH JCDRI/M YB/G 321 2:35
HKMLVNQOSG/G
ARN 2035 TH 2:35
COMMENTS-
1.LHR ARN - MEMBER OF ONEWORLD
2.LHR ARN - DEPARTS TERMINAL 5
3.LHR ARN - ARRIVES TERMINAL 2
4.LHR ARN - 9/ NON-SMOKING
5.LHR ARN - ET/ ELECTRONIC TKT CANDIDATE
6.LHR ARN - CO2/PAX* 143.74 KG ECO, 143.74 KG PRE
(*):SOURCE:ICAO CARBON EMISSIONS CALCULATOR
CONFIGURATION-
321 C 20 M 175
>"