So Im finally reading darksied war which is near the end of new 52, what doesn’t make sense to me is how Oa is mentioned by Hal. Oa has been destroyed for quiet a while, so that means this has to take place before that right? I mean Oa was destroyed in the lights out story arc around issue 23. So this is a major error or it has to take place before that, because Oa isn’t rebuilt until the end of the rebirth series.
can anyone answer this?
Yeah, there are lots of errors in continuity like that, especially between editorial groups. This one is pretty minor, as it can be written off as Hal still thinking of Mogo, sitting among Oa’s ruins, as being “Oa.” (But you’d think he’d be more hurt about Metron killing Mogo than “attacking Oa”…)
For contrast, in the same Godhead issue where the attack happens, a Red Lantern removes her ring and comes to no harm, something that was previously only accomplished by Supergirl flying in to the Sun. And I think its in Darkseid War where Orion meets Wonder Woman as if for the first time, despite their mutual history with Milan.
Darkseid War was pretty far out of line with most of its members’ titles at the time. No GordonBatman, no depowered/ outed Superman, WW in her classic outfit, etc.
This specific issue confused me for a while. It seemed like Oa was referenced in every other non-Green Lantern title at the time it was destroyed, which always threw me off.
I wasn’t reading Green Lantern at the time, so I can’t speak to that specifically. However, I know that continuity was very difficult to maintain during the New 52 as a whole. As @keath mentioned, there were issues between editorial groups. My personal theory is that this is partially down to the business strategy of New 52. Every few months, DC would cancel books or drop editorial/creative teams from books depending on how they were selling or how they were being received. This lead to storylines and characters being quickly dropped which also lead to creator dissatisfaction and some resignations. This was, possibly, the biggest flaw of the New 52 era and made it extremely difficult to maintain a set continuity.
The way I personally got around it was headcannon retcons. It seemed to me that Justice League was the flagship book of the DC Universe at the time. So, I prioritized that and then made everything else fit around it using mental gymnastics. It’s challenging, but… not impossible. There was a lot I did like about the New 52, but it does feel like continuity was less of a priority then.
I just went with the assumption that the two complete reboots of reality that occurred (plus the year of time travel alternate timeline that blipped away) could explain any apparent conflicts. 
In my head, that’s where Sheba went, why Red Robin could fly and Kid Flash could float, how Doctor Occult reappeared without resurrection, how the Eiffel Tower is still standing, and so on and so forth…