How do Opalescence and Humility interact?
This is actually directly addressed in the rulings for Opalescence:


  This is the current interaction between Humility and Opalescence: The type-changing effect applies at layer 4, but the rest happens in the applicable layers.  The rest of it will apply even if the permanent loses its ability before it's finished applying. So if Opalescence, Humility, and Worship are on the battlefield and Opalescence entered the battlefield before Humility, the following is true: Layer 4: Humility and Worship each become creatures that are still enchantments. (Opalescence). Layer 6: Humility and Worship each lose their abilities. (Humility) Layer 7b: Humility becomes 4/4 and Worship becomes 4/4. (Opalescence).  Humility becomes 1/1 and Worship becomes 1/1 (Humility). But if Humility entered the battlefield before Opalescence, the following is true: Layer 4: Humility and Worship each become creatures that are still enchantments (Opalescence). Layer 6:  Humility and Worship each lose their abilities (Humility). Layer 7b: Humility becomes 1/1 and Worship becomes 1/1 (Humility).  Humility becomes 4/4 and Worship becomes 4/4 (Opalescence).


This all sounds very confusing, but basically what it is saying is that it depends on which one entered the battlefield first. These types of effects are applied in a "Layer" order. When 2 things would happen in the same layer, they are applied in the order that the effects were created.