A very common and good question. High oxalate vegetables contain many important nutrients beyond just calcium and should definitely be part of a bone healthy diet. It is true that oxalate can adversely affect the absorption of calcium. However, when the 1200 mg. calcium recommendation was set it took into account that calcium absorption varies significantly according to the food so you don’t have to worry about absorption rates and can just focus on the goal of 1200 mg. calcium per day.
The recommended amount of calcium is based on the assumption that you will consume foods with varying degrees of calcium availability and that you will absorb less calcium from some foods than others. It could be a problem if the only high calcium foods you eat are high oxalate or if you consume extreme amounts of high oxalate foods but if you are consuming a diet with a variety of high calcium foods beyond high oxalate vegetables then it shouldn’t be a problem. If you have had gastric bypass surgery or have a dysfunctional gi tract, where nutrient absorption is an issue, then you might need to pay more attention to this and work with a dietitian to address. Hope this helps.