Mike is able to support a stay-at-home wife, six kids, and a full-time housekeeper on a staff architect's salary. He also has a huge house, can afford vacations, and the kids have their own bikes. Since Mike was a widower, he may have got money from his late wife's life insurance policy. The house he lives in in the show's pilot is also different so he may have had extra money from selling it, and Carol sold her old house as well; she could also have money from her late husband as well.
A couple seasons prior to Greg moving into the attic, Mike says that he'd have to be two feet tall to live in there. It could have been renovated in that time span.
Much has been made about Oliver in the meta-sense but not in the social sense, as he at a very young age moves to live with his extended family while his parents go on a dig to South America.
Oliver wholeheartedly believes that he is bad luck (and that all started because he was overeager to show his new family how worthwhile he was)
In The Brady Girls Get Married Wally, a complete stranger, manages to sends Marcia flowers by asking the cashier where she worked and then gets her home address from her company. Everyone's lucky Wally ended up being a nice guy and not some crazed stalker.
