Fabric has a lot to do with it. If you bought oversized in thick cotton or natural fabrics with some structure, they'll hang better and wrinkle less than ones that are very lightweight or have synthetic blends that move too much. Before you change how you style them, check what fabric exactly is what you bought, because a quality oversized tee looks neat even when it's big.
Now, how you wear it also matters quite a bit. What works is tightening up the rest of the look: if the tee is wide up top, some more fitted pants or a belt that defines your waist compensate for the volume. A lot of people make the mistake of wearing oversized with baggy pants too, and then it really looks like you're lost inside your clothes. Try tucking it in on one side at the waist, or just wear it untucked but with something that defines your silhouette down below. Another option is to pair it with something more structured on top, like a jacket or an open shirt.
And a practical detail: iron your tees as soon as you take them out of the wash, even if they're cotton. An oversized piece without wrinkles looks completely different. If the fabric still looks loose or undefined even after ironing, it probably wasn't the best purchase, but give it a shot with this method before you write it off.