Wearing a simple white Oxford shirt with the collar button unbuttoned and without a tie is a great way to slightly reduce the formality of a suit without looking unconventional or too casual. We are in the golden age of casual wear and that has been a blessing for everyday attire. Once, men had to fight all year round in their undershirt, collared shirt and suit jacket, sweating from the summer heat, but those days no longer exist. Now, it's perfectly acceptable to wear a t-shirt under the suit, and in fact, in summer, it's even preferable.
It's great and effortless and has just the right amount of DGAF. If you're going out after a day at the office, change your collared shirt to something more casual, like a white t-shirt. The white t-shirts that look best on most men are straight cut, fitted and round neck. While some consider a white shirt boring, think of it more like a canvas for the rest of your outfit.
And then there's Brad Pitt, who does press in Japan, wearing layers of white, including a white blazer, a white buttoned, but not fastened, and a white t-shirt.