Make sure to enunciate when you cast this.

Attacking creatures are only considered unblocked after blockers have been declared. Before then, attacking creatures are neither blocked nor unblocked.

Noble Ox’s enters-the-battlefield ability causes all unblocked creatures attacking you to become blocked by Noble Ox, regardless of whether Noble Ox can normally block that creature.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.