In the underground caverns of Onu-Koro, hard work is rewarded with great wealth, at least according to the Principle of Prosperity. For the effects of the Field Spell, I interpreted “hard work” as “putting monsters in the GY”, which is usually a pretty decent indication you’re doing things, and “wealth” as the resources of both LP and cards. So the first effect trades the proof of your work, the monsters in the GY, for wealth in the form of LP, and the second directly trades that amassed wealth for cards.
There are several extra balancing factors to the draw effect, given that it can potentially draw up to 3. First, it only works when you have higher LP, so you have to do some healing and/or damage before using it. Second, you need to send an EARTH monster from your hand or field to the GY, mainly to downgrade the level of advantage you get and to make the effect a bit more conditional, but also as setup for LP regeneration. Third, if you pay so much that you are now behind in LP, you lose an equal number of cards to what you drew, which technically works out to a -1 but is still fairly good since you can freely put anything from your hand into the GY. Fourth, it locks you into only summoning EARTH monsters for the whole turn, just to be extra sure it doesn’t get randomly abused. A proper Onu-Koro deck is mostly okay with this restriction anyway.
It follows that, as an Onu-Koro player, you want to establish a stable loop of putting EARTH monsters in the GY while building your board, shuffling them back to gain LP, and potentially drawing additional cards to further strengthen your position. Now we’ll take a look at how other cards contribute towards that goal.
Whenua essentially acts as an overseer of the “work” you perform, providing small rewards in real time for every EARTH monster you put into the GY. On summon, he also helps you solve your current problems by learning from the mistakes of the past, or in non-lore terms, searches a low-level EARTH Warrior that isn’t in your GY yet. Of course, putting stuff back with Onu-Koro expands your search range here.
The Kanohi Ruru, Mask of Night Vision, doesn’t really have a main focus that synergizes with the Onu-Koro deck much – it just reveals Set cards, because that’s the mechanic that historically fits night vision best (for the record, hand reveals would be mind reading). It does also inflict some damage as a bonus, which could at least help achieve the necessary LP difference to use Onu-Koro to its full potential.
The search effect on Whenua is of course meant to fetch Onu-Matoran, and as the only one implemented so far, the choice will probably fall on Taipu a lot of the time.
Always eager to come to the aid of his friends, the stronkest of Matoran will jump from your hand to the field in just about any situation, but by being a bit clumsy about it he’ll mostly lock you out of attacking for the turn. Don’t ask me how he does that.
On his way out, the helpfulness continues, with a generous stat boost given to whichever friend remains around. Due to the largely irrelevant nature of stats, the greatest use for this is actually chain blocking.
A potential hole in our whole strategy of continuously shuffling monsters from the GY back to heal and draw is that we will eventually run out of Spells and Traps, as those are not recycled. It may not matter a lot of the time, but still, to our rescue comes the wise Toa of Earth.
Indeed, Onua gives us the ability to return any card from the GY to the Deck, assuming the trigger of a monster being sent from hand or Deck to GY applies. And beyond simple recycling, he comes with the benefits of working on both GYs, being able to place the card either on top or bottom of the Deck, and healing you if you return a monster with lower ATK than him. This means he can put your own Spells/Traps just a draw away, place something dead on top of the opponent’s Deck, take something they just sent to the GY away before it can be used, or simply be used for LP regeneration to enable Onu-Koro.
The Kanohi Pakari, Great Mask of Strength, also plays into these LP games. The bonus ATK on Onua makes it so you stand to gain more LP from his effect, and a high-ATK piercer is a pretty reliable way to damage your opponent and help establish the desired LP difference.
Best of Test
The greatest strength of Onu-Koro decks is the ability to replenish resources, both cards and LP, over and over again. Even if you can’t kill your opponent quickly, you can probably survive long enough to outlast them if things go reasonably well, though most of the time there isn’t a need to drag things out too much. Here‘s a sample Gouki-based decklist to get you started, slightly more updated versions are included with each BCOT release.