If Humility and Opalescence are both on the battlefield, what are their power and toughness?

This is a classic layers interaction. Both Humility and Opalescence generate continuous effects that must be evaluated in the correct layer order. Opalescence (layer 4) makes enchantments into creatures with power/toughness equal to their mana cost. Humility (layer 6) removes all abilities from creatures, and (layer 7b) sets all creatures' power and toughness to 1/1.

Because layer 4 (type-changing) is applied before layer 6 (ability removal) and layer 7b (P/T setting), Opalescence's effect first makes both enchantments into creatures. Then Humility strips all abilities from all creatures (including Opalescence's own ability and Humility's own ability, but those effects are already generated and continue to apply — see CR 611.2c). Finally, in layer 7b, Humility's P/T-setting effect makes all creatures 1/1.

However, since both Humility and Opalescence each generate a layer 7b effect (Humility sets to 1/1; Opalescence sets P/T equal to mana cost), these two layer 7b effects are ordered by timestamp (CR 613.7). The one that entered the battlefield later applies last and wins. If Opalescence entered last, both are 4/4. If Humility entered last, both are 1/1.

Concrete example: You control Opalescence (entered turn 3) and Humility (entered turn 5). In layer 4, both become creatures. In layer 7b, Opalescence's effect (timestamp 3) sets them to 4/4, then Humility's effect (timestamp 5) overrides and sets them to 1/1. Both end up as 1/1 creatures with no abilities.

If Opalescence had entered after Humility, its layer 7b effect would apply last, making both 4/4. In all cases, Humility's layer 6 effect removes abilities in layer 6 regardless of timestamp, so neither permanent retains its printed abilities. (CR 613.1, 613.7, 611.2c)