Sharp estimates of integral functionals on classes of functions with small mean oscillation

We unify several Bellman function problems into one setting. For that purpose we define a class of functions that have, in a sense, small mean oscillation (this class depends on two convex sets in ${\mathbb{R}}^2$). We show how the unit ball in the $\mathrm{B}\mathrm{M}\mathrm{O}$ space, or a Muckenhoupt class, or a Gehring class can be described in such a fashion. Finally, we consider a Bellman function problem on these classes, discuss its solution and related questions.