Apple - How do you type Theta on a Mac?

Γο το Συστεμ Πρεφερενψεσ/Λανγθαγε & Τεχτ/Ινπθτ Σοθρψεσ, σεαρψη φορ Γρεεκ, αδδ ιτ. Νος υοθ μαυ τυπε θ φρομ κευβοαρδ.

Go to System Preferences/Language & Text/Input Sources, search for Greek, add it. Now you may type u from keyboard.

Or you may just use Find and replace function in Edit menu to change ø to θ afterwards.

It looks like no one has mentioned the Special Characters window.

1. Press Command ⌘+Control ⌃+Space to bring up the Special Characters window.
2. Type theta into the search bar.
3. Your theta should be there:

As pointed out in this post, you can add keys to the press and hold key feature.

Edit the file "/System/Library/Input Methods/PressAndHold.app/Contents/Resources/Keyboard-en.plist", and add the following code:

By default, there is "no Roman-Accent-t" section, I coppied mine from "Roman-Accent-s." You could add θ to any key you like (e.g: "o") by modifying the appropriate section.

    <key>Roman-Accent-t</key>
    <dict>
            <key>Direction</key>
            <string>right</string>
            <key>Keycaps</key>
            <string>t θ</string>
            <key>Strings</key>
            <string>t θ</string>
    </dict>

Note: Case sensitive. "Roman-Accent-T" is NOT the same as "Roman-Accent-t"

Caution: This is a system-wide change. Edit carefully or make backups.

Now, when you press and hold 't' this appears:

Then click θ or press 1 to have a "θ" inserted into your document.

I did not have to relog after making edits.