Oddric’s ability triggers at the beginning of each combat, not just combat on your turn, whether or not any creatures you control have any of the listed abilities. If a creature gains one of the listed abilities before Oddric’s triggered ability resolves, perhaps due to another ability that triggered at the beginning of combat, then creatures you control will gain that ability.

The set of creatures affected by Oddric’s ability and how they are affected is determined as the ability resolves. Creatures you begin to control later in the turn won’t gain any abilities or cause creatures to gain new abilities, and the abilities gained won’t change even if every creature that normally had the abilities leaves the battlefield.

If one of those creatures has one or more variants of the listed keywords (for example, islandwalk), creatures you control gain those specific variants.

Gaining changeling or devoid works exactly how you would expect it to unless you’ve spent too much time looking at the Magic rules.

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.