Big O is the upper bound, while Omega is the lower bound. Theta requires both Big O and Omega, so that's why it's referred to as a tight bound (it must be both the upper and lower bound).
For example, an algorithm taking
Omega(n log n) takes at least
n log n time, but has no upper limit. An algorithm taking
Theta(n log n) is far preferential since it takes at least
n log n (Omega n log n) and no more than
n log n (Big O n log n).