Typing macron using the Kotoeri Romaji mode Given that I often type in Japanese or romanized Japanese, my default keyboard layout is the Romaji mode of Apple's Kotoeri input method. But I keep running into an inability to type e.g. ō without switching (using the mouse) to U.S. Extended.

My search skills both on Google and on this site have yielded no ways of typing the ¯ character anywhere within Kotoeri. Has the entire Internet missed this, or am I just so stupid that everyone's figured it out and it wasn't even complicated enough to warrant how-to articles? Or is it impossible?
While reading your question, I happened to stumble upon the (imho) easiest option:

Simply uncheck the "Romaji" checkbox under Input modes in your Japanese/Kotoeri pane in the System Preferences. Then you can choose an easy keyboard shortcut for switching exclusively between U.S.Extended (with the āēīōū capability) and the Hiragana Japanese mode.

Since I use a Japanese Apple Keyboard which includes the 英数 (Romaji) and かな (Kana) keys next to the space bar, this is ideal for me, but you could just use any other shortcut as well.