Obsidian rock is a pretty common ressource on the grand hills, just bring your argy on top of the mountain and there are alot of rexes on the grand hill but there are no enemies on the very top. Once you are done gathering obsidian you can get rid of any unwanted ressources to get rid of the extra weight and make sure to give the obsidian to your argy for the way back for the 50% weight reduction
DONT USE DOED!! Ankylo or Phoenix is the best by far, this is from personal experience. Doed gets you only stone from MOST the obsidian around, on some maps there is another kind which has metal too, that is the ONLY kind a doed can even harvest at all.
PHOENIX IS BADASS!! mine was 37 b4 tame, and now has replaced my quetz cause it has a weight over 1200, and it can harvest all the same stuff as an Ankylo, not as good, but not much worse tbh.
To farm the most obsidian in the game take a god dunkleosteus in the water above volcano on rag and look for a ravine it’s filled with soo much obsidian and dunke has the most gathering efficiency and weight reduction it’s by far best method out apart from stryders -T8
Have tried anky mantis and dunkle. Tested on land and water for both mantis and anky and dunkle is by far better on official. Island hunter cave dunkle I get 5k a run on 1x. Barely 2k with the other 2. Put dunkle on top as best gatherer please.