MOC orders have many limitations and special requirements that make them incompatible with "One Cancels All" groups, Johannes. And internally, bracket orders are basically OCA groups. Take a look at "". NASDAQ MOC orders, for example, cannot be cancelled after 15:50 EST.
Are you trying to participate in the Market on Close auction (e.g. you really need the MOC order type), or are you simply trying to close the position automatically at the end of the trading session? We successfully use bracket orders with an additional third child order that closes the position a few minutes before the market closes. Basically just a MKT type order with a time condition.
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