Atua, atu'a

1. Lord, God: te Atua ko Makemake, lord Makemake. Ki a au te Atua o agapó, I had a dream of good omen last night (lit. to me the Lord last night). 2. Gentleman, respectable person; atua Hiva, foreigner. 3. Atua hiko-rega, (old) go-between, person who asks for a girl on another's behalf. 4. Atua hiko-kura, (old) person who chooses the best when entrusted with finding or fetching something. 5. Atua tapa, orientation point for fishermen, which is not in front of the boat, but on the side. Atu'a, behind. Vanaga.

God, devil. T (etua). P Pau., Ta.: atua, god. Mgv.: etua, god, deity, divinity; to be wicked, to be full of wickedness. Mq.: etua, god, divinity. The comprehensiveness of the definition, and the same is found in the Maori, is a question of orthodoxy, merely a matter of the point of view. Of far more moment in our studies is the vowel variety of the initial syllable. Atua: Maori, Mangaia, Tahiti, Hawaii, Tongareva, Rapanui, Paumotu, Samoa, Futuna, Uvea, Niuē, Aniwa. Etua: Marquesas, Mangareva, Rapanui. Otua: Tonga. The Rotumā oiitu is probably referable to aitu. Churchill.

Mq.: atua, the fourteenth day of the moon. Ma.: atua, id. Churchill.

I suggest Otua is a wordplay involving 'o tu'a (the back side). Beyond full moon the waning moon will arrive and the 'season' can be referred to as the 'back side'. The end of waning moon is depicted as a turned around 'person' with the 'back side' forward:

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Ca8-22 Ca8-23 Ca8-24 Ca8-25 Ca8-26
Ca8-27 Ca8-28 Ca8-29
Tu'a

1. Back, shoulder, tu'a ivi, shoulder blade; tu'a ivi more, lumbago; moa tu'a ivi raá, 'sun-back chicken': chicken with a yellow back which shines in the sun. 2. Behind (a locative adverb, used with i, ki, a, o, etc). Tu'a-papa, pelvis, hips. Vanaga.

1. Behind, back, rear; ki tua, after; o tua, younger; taki tua, perineum. 2. Sea urchin, echinus. The word must have a germ sense indicating something spinous which will be satisfactorily descriptive of the sea urchin all spines, the prawn with antennae and thin long legs, and in the Maori the shell of Mesodesma spissa. Tuaapapa, haunch, hip, spine. Tuahaigoigo, tattooing on the back. Tuahuri, abortion; poki tuahuri, abortive child. Tuaivi, spine, vertebræ, back, loins; mate mai te tuaivi, ill at ease. Tuakana, elder, elder brother; tuakana tamaahina, elder sister. Tuamouga, mountain summit. Tuatua, to glean. Mgv. tua: To fell, to cut down. Ta.: tua, to cut. Mq.: tua, to fell, to cut down. Ma.: tua, id. Tuaki, to disembowel. Ma.: tuaki, to clean fish. Tuavera, the last breadfruit spoiled by the wind. Ta.: tuavera, burnt by the sun. Churchill.

The back side (tu'a) is the temporal opposite (autumn, waning) of the front side (ra'e). It is the opposition between 'evening shadow' (ata ahiahi) and 'morning shadow' (ata popohaga).

As to Churchills remark oiitu = aitu, I cannot resist the temptation to quote:

"According to Alfred Patterson, the hare tongi were built on [the island of] Hare as places for people to hide from spirits (aitu) which came in from the sea. 

E hakamuni ni aitu takapo tai means 'to hide from spirit groups from the sea'. The idea evidently was that people could be seen in normal houses with open sides, whereas they could not be seen by the spirits when the roof came down to the ground. 

At [the island of] Touhou, according to Patterson, the people could be protected from the spirits by the ariki priest who resided there but at Hare they had no such protection. 

He also stated that the aitu came in from the sea during the middle part of the day, about 11 a.m. to 2 p.m. Hence, women working in the puraka cultivations in the outer islands took care to return to Touhou before the dangerous period started." (Kapingamarangi)

Spirits or gods or devils - same thing: 'full of wickedness'. The full moon 'season' is equal, structurally regarded, to 'full sun' - the 'season' around midday (when aitu come in from the sea).