It's not the oxalic acid, it's the calcium:
Potassium oxalate did not influence iron absorption in humans from a kale meal and our findings strongly suggest that OA in fruits and vegetables is of minor relevance in iron nutrition.
In short, calcium is extremely digestible in chloridric acid (some people even use it to polish marble)...
Consume equal parts spinach and orange/lemon juice for best absorption of iron. It’s okay to drink more orange or lemon juice than that, but it might be more costly.
The presence of vitamin C is most important when eating foods that also contain inhibitors of iron absorption. Orange juice and spinach are a great combination.
The promotion of iron ...
According to The Vegan Society's iron leaflet
relying on non-heme sources of iron gives the human body control over absorption sufficiency, by allowing it to increase uptake to suit its needs.
So this should not be seen as a problem.
Oxalic acid is broken down by cooking, and, besides being a source of iron
Spinach has a lot to offer nutritionally: ...