Since you're on XDA we'll assume rooting and customization are important to you which pretty much makes the AT&T version of the Galaxy S5 a non-starter.
The OPO is closer in size to the Galaxy Note 3 than the S5 though, so you'd need to decide if the size is right for you to begin with.
GS5 has a great camera so that might factor into your decision too. We have yet to see any head to head image comparisons between GS5 and OPO but it's my understanding the OPO uses the same camera as the Oppo Find 7/a. I *have* seen a head to head comparison between the Find 7a and GS5 and the Samsung images were better, so it's reasonable to assume similar results may be seen with the OPO. I look forward to seeing a good shootout comparison, hopefully soon.
Since you're on AT&T it's worth noting that OPO only supports two out of AT&T's four LTE bands. Someone will correct me if I'm wrong but I assume the AT&T GS5 supports all 4 AT&T LTE bands, meaning it may perform better on AT&T's network in high traffic areas where those other two LTE bands are in use.
If you're eligible for a subsidized phone on contract from AT&T then the GS5 is cheaper. Otherwise at full price the OPO is cheaper.
Finally, OPO has no SD card expansion slot, the battery is not removable and it's not waterproof, so the GS5 wins in those categories too (except the lack a 32GB version of the GS5 is lame despite the SD card slot). The workarounds are to buy the 64GB OPO variant, carry a portable charger and don't drop it in the toilet. Probably not deal-breakers but still worth noting.