The AHD agrees with the OED: commoner and commonest are standard as the comparative and superlative, respectively, of the adjective common.

What is the superlative form of common?

For example, the correct comparative and superlative forms of common are commoner and commonest.

Do we say most common or commonest?

According to Swan (my grammar bible), common can equally be used with -er and -est as well as more and most. And it's most common to say 'most common' (3,150,000). 'Commonest' only has 101,000. Comparatives and superlatives for words of three or more syllables invariably use more and most.