Loosen the setting, not necessarily the fit
Try your shirt with straight jeans and a pair of shoes you already walk in comfortably. Open the collar to the point that feels right, then check whether the cuffs look better fastened or folded back once. Those changes can be enough to move an office shirt into a weekend outfit.
The fabric makes a difference. A very crisp shirt keeps a structured outline, while a softer one tends to settle around a waistband. Neither is automatically more casual. Look at how the shirt sits after ten minutes of wearing it, rather than only when freshly arranged.
Use it as a light outer layer
Over a vest or a simple sleeveless dress, an unbuttoned shirt can offer coverage without the weight of a jacket. Check that the armholes leave room for the layer underneath. If the back pulls when you reach forward, a different shirt will be more useful than leaving more buttons open.
For a loose shirt over wide trousers, try a shorter base layer underneath. The opening gives the outfit a visible centre even when neither piece is fitted. Let the shirt hang naturally; constantly adjusting a complicated knot usually makes an everyday outfit less convenient.
Check it in the light you will wear it in
White fabric can look different beside a window than in a bedroom. Check the front, the back and the outline of anything underneath. Choose an underlayer for comfort and the coverage you want, rather than assuming all white shirts behave alike. Finish by testing your bag strap: delicate fabric may bunch beneath a heavy tote.
