Yeah ANZ is a bit confusing & cumbersome compared to ASB in some ways. The OMCA account is kinda the same as the ASB cash management account. Except unlike ASB where it is a single account in your name, from what I can make out , ANZ is a pooled account i.e all customers money is in the same account and your individual balance is tracked by your client number. The OMCA is where the trades are settled i.e. once the sale completes the money is deposited or withdrawn from this account. Normally 2 days after your trade depending what market you’re trading. Note the OMCA is multi currency which is hugely beneficial when trading say Aussie shares as you can settle in Aussie dollars and don’t continually get stung with exchange rate fees. (ASB also have other currency accounts but you need to ring them and set them up where ANZ have them ready to go)
The trading account tracks your pre-settle balance. Same as what’s in the top right corner of the ASB securities screen. This is the ‘how much you will have once all your trades are settled’ account. It’s more of a fund tracker than an actual account.

If your actively trading then hands down use ASB as their system is much faster (in most cases instant). The benefit of speed hugely outstrips the benefit of the cheaper brokerage from ANZ. Time is money when trading and ANZ require a broker to place the trade for you. They may be on the loo or at lunch or chatting to their mates about the weekend. I have waited 15 minutes for a trade to get placed with ANZ before and then it took them another 2 hours to process it so the money was unavailable in my trading account. This quite frankly is piss poor. ASBs appears to be much more automated so money can be spent within a few seconds of your last sell.

Transfering money from your OMCA is the same as transferring it from bank to bank as ANZ Securities appears to be completely separate from ANZ banking. Transfers will be done ‘overnight’