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The child orders are placed in an OCA group, so generally speaking, only one order will execute and the others will be cancelled.

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However, OCAs are not magic, and there are rare situations where more than one order is filled. A case I could imagine is where you have an OCA with two stop (or stop-limit) orders to catch a breakout in either direction from a narrow trading range. Conceptually at least, there is a possibility in a frantic market that it could move one way (triggering one order) and then rapidly reverse, triggering the other before IB has had time to react and cancel it. In practice I suspect this is unlikely nowadays, because for most exchanges these days stop and stop-limit orders are not native but are simulated by IB. So provided the first order's fill notification reaches IB quickly enough, it should be able to avoid triggering the second order when it's trigger price is posted.

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An example of an exchange where stop-limit orders are native is Globex, according to IB's exchange listings.

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My point about being careful was literally, just be careful! Don't make any unnecessary assumptions. For example, if you put a 'good till date' on the stop-loss order, make very sure through testing that it will be honoured in a timely fashion so you don't end up with your stop-loss order still active at the same time as your position-closing order is active. Whether this would be a good approach or not, I don't know offhand.? The easiest way is probably just to modify the stop-loss order so that it is instantly triggered, provided there's little or no chance of what I described above happening.

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The way I do it, cancelling existing orders and then placing a position-closing order only when all cancellations have been confirmed, still has a finite risk from the connection dropping or the exchange crashing in those few milliseconds, but that would be seriously bad luck: but anyway it's impossible to remove all risk.

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You should never take something you read about the API at face value, for example "I recall seeing a posting claiming you can't add a third child.".

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It doesn't matter who posts it, we all make mistakes, so you always need to do your own verification.

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As it happens, what you recalled is untrue: you can certainly add a third child order. I just did it: parent limit order, child limit target order, child stop stop-loss order, child market 'good after time' order. As it happens that particular bracket order was rejected because the contract I was using (the DAX future FDAX SEP 20) has an absurdly high initial margin requirement, and my paper trading account doesn't have enough funds in it. But an earlier attempt, using an additional MIT target order instead of the timed market order, with a different contract (ZU0) worked fine.

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What I also discovered is that MOC orders are not accepted at the exchanges and for the products (futures) that interest me. The description of MOC orders on IB's website – see ? – includes a link to a list of supported exchanges: this includes DTB (which is where the DAX futures are), but MOC's weren't allowed with the FDAX SEP 20 contract, so it's all a bit hit-and-miss.

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