How do I not look scruffy in oversized t-shirts?

Don't make the mistake of thinking that oversized = shapeless. An oversized tee needs a fit that has structure anyway: pick pieces that drape well, maybe a bit oversized but not like a garbage bag. The fabric matters a lot - quality cotton, linen, materials that hold their shape instead of sagging. And if it's too wide in the shoulders or length, get it altered: a straight, even hem makes a huge difference compared to a wavy edge.

The rest depends on how you style it. An oversized tee with slim or straight pants (not baggy) creates contrast and the look comes across as intentional, not lazy. If the color is dark or neutral it works better than random prints. Pair it with more structured pieces - a belt, a well-cut jacket, clean shoes - and you're sending the message that you picked it on purpose, not that it was the first thing you grabbed from the drawer.

Small details change everything: an oversized tee tucked in slightly at the front, or tied to one side, breaks up the monotony. Avoid pairing big with big (oversized tee + pants + jacket all loose at the same time), because that's when you actually look lost. Balance is the key, not necessarily being tight-fitted.