So forget relying on a joint checking account, he says.
"Never merge all of your assets into your significant other's account," O'Leary says. "Really bad idea."
That's because you need to build a credit score and financial history for yourself, he argues.
"The reason you want your own account, and particularly your own credit card, is if you pay it off every month — that's the first way you start to build a credit score," O'Leary says, referring to the metric that determines what kind of interest rate you'll pay on things like mortgages or auto loans. "That makes things cheaper for you later in life."
O'Leary advises each person maintain a personal checking account, and says then you can open one together for shared expenses like food and rent.
"Here's the methodology for marriage when it comes to bank accounts: Each person has their own, and then you create a joint account," he explains. "You can always have a joint card, you can also have a joint account, but you need to maintain your own financial identity forever."
The joint account should only have enough money to "maintain your living expenses," while the bulk stays in your separate accounts, he says. If one partner makes significantly more money, "you might choose to divide living expenses accordingly, but both parties should still have their own bank accounts."
Twenty-eight percent of married millennials already keep their finances separate, according to a 2018 report by Bank of America, compared to 11 percent of Gen Xers and 13 percent of Baby Boomers. Nearly 20 percent of millennial couples surveyed didn't even know how much their partner makes.
When it comes to money and love, the most important thing to do is get on the same page.
"Make money something you think about together," he advises. "It's going to be part of your life forever anyway, you might as well talk about it early on."