“Then \Omega is italics by default and \varOmega upright. Package siunitx uses \upOmega, if it is available to initialize the symbol for math-ohm, otherwise \Omega is used as \text{$\Omega$}, which results in an italics Ω. Whereas \mathrm{\Omega} would have given an upright Ω. ”
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Using siunitx, \ohm results in an italic Omega
