Both are at full power and are using current versions.
Nami has her Perfect Climatact, which can control different aspects of the climate, including mirage and lightning strikes. Orihime can use her innate powers (well, hair-clip powers) to form barriers, heal and attack. She may also have some measure of endurance, as she took a bitchslap from Yammy and survived. unlikely though.
So I go into the battlefield and convince them to stop fighting, and come home with me, where we have lots of sex. Then they call all their friends to come over for you guys. So shotgun people now.
Gender: Male Location: In Luna's mane, chasing STAAARS!
Unfortunately for you I was in the shadows and blowed your brains out with my shotgun. With you gone I took the two (plus Matsumoto Rangiku and Nico Robin) to my place
No. Nami has no chance of winning.
Orhime is so fast she can follow Ichigo and Ulq fighting.
Orihime's forcefield can block casually skyscraper destroying attacks.
Orihime can reject the very existence of the opponent she is fighting and any equipment they may posses.
She is one of the most powerful characters in all of Bleach.
__________________ Psychology is a weapon against probablity, so never lay down your cards.
Orihime has the potential to win this fight in a stomp, but her current character incarnation would get mopped up by Nami. She's got more battle experience, and the souped-up Climatact is deceptively powerful.
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-"You have the best problems, Scenario. You really do. haermm"