Is OATH worth it? Which dolls are worth oathing?

IMO it is worth it, really, it worth the 580 gems you spent, or even more. You can oath any dolls that you like even 2 stars if you want to, either to make her stronger or show everyone how much of a desperate weeb you are by oathing a LOLI, but you should just use it on 4-5 stars (M14 is an exception because she is so OP).

As for the AR team, M4A1, AR15, and RO635 is really useful. 404 Team, probably, UMP45 and G11.

For the other type of dolls, I would suggest the following(just suggestions)
AR: OTs-14, Type97, G36, Soppo, M16A1(powerful masochist)…
RF: Wa-chan, Lee, IWS(OP Raifu), NTW-20(Nuking Bosses), Springfield(obviously)…
HG: All HG in Tier One, M1895, PPK, Noel(For that borderline-hentai damaged art)…
MG: PK, MG5, LWMMG, Negev(Lolies :v)…
SG: Ephelt (Her stats and buffs are phenomenal), KSG, SAT-8…
SMG: Thompson or G36(Ignore MP5), Ingram or Skorpio, the UMP sisters, Vector, SR-3MP(if you want a DPS gun), and IDW da nyaaaaaaa (ignore this)…

But hey, you play the game, right? Oath any dolls you want. Heck, I oath IDW just for meme's sake, I oath C96 just because of some H*ntai that I read online.

Lol. Gotta always make an IDW DA NYA meme anytime possible.

*IDW DANYAAAAing intensifies*

Yeah, oathing makes a T-Dolls a lot more powerful, but personally I try to oath for weeb purposes. Just go oathing you waifu-rank list.

SOP-II best grill. Fite me.

WA 2000 is up there.

Just be aware that oathing does not increase the RoF. Because of this, I generally use them on ARs, MGs, or offtank SMGs like SR-3MP. RFs aren't entirely worth it since they just end up overkilling and it's a bit of a waste, tho those that are used for killing bosses for sure are; that extra damage goes a long way.

G11 is worth oathing, because sleepy girl is best girl.
Moreover, her active skill scales with any damage increases, which oathing does.