They are not true inverses of each other because both supply and demand curves moves to the right daily in the first few weeks, leading to a gradual price increase. That is, both supply and demand are rising.

Demand rises faster (players grow and need more oil to grab) than Supply (oilers don't grow as fast because top players rarely play oiler), but both are rising. This results in a price overall increase.

Oil is definitely elastic, otherwise people wouldn't be stocking and speculating on it.

Just because price vs qty sold/day aren't inverses of each other doesn't indicate a good is inelastic. There is increasing demand for oil despite the increasing oil prices (which would cause a demand drop) because the additional income from land gains resulting from grabbing is far superior. The increasing demand for oil due to external factors (for grabbing) far exceeds the drop in demand caused by oil price rise.