Since this is the OU forum, it's worth pointing out that OU is a smogon creation so I think you'll only find OU tournaments here. Outside of smogon tiers, plain 6v6 singles is pretty dead outside of little battles between small friend groups, although 3v3 singles (aka battle stadium singles) is pretty popular in japan.i'm pretty new to the tier and non randbats formats in general and i think the best way to improve is by playing in tournaments as much as possible. i'm pretty familiar with the tours hosted within smogon - heck, i'm a host for ost lol - but have almost no knowledge of tournaments hosted in outside communities. where are some other places i could get playing experience under my belt?
However if you're looking for something completely different that has an established tournament structure and is completely disconnected from smogon, why not try out VGC?