Spinach (like rhubarb, chard, beets) in any meal without calcium (as in dairy, bones, etc) can raise your oxalate to near fatal levels. Simultaneous calcium makes the oxalate unabsorbable, but the two must travel the gut simultaneously.
Worst: alternate days lots of dairy (enrich blood calcium) then next day lots of spinach (oxalate gets into hyper-calcium blood, produces Ca-Ox crystals, insoluble, = kidney damage). Best case: have the Ca-Ox precipitate crystals form in the gut, never to get into the blood stream. No kidney damage.