Typically any magatama with "Odachi" in the description? Not sure what you're asking about here.

it all depends. you have to be more specific.

Different leveled characters can equip different gear as the need change, but as a general rule of thumb,

MAX Affinity 1st, which for a low level character, magatamas such as Coloured Carp might come in handy.

Then there is Skill Force Magatama, such as Dusk Magatama to increase attack power of skills above a certain rank.

Last, but certainly NOT least, elements. Slice is good for Odachi in general (exceptions apply).