Never.
C° is always C diminished.
C major seventh can be signed with a triangle after the C. Bear in mind that half-diminished is signified by a circle with a diagonal line through it.
Cmaj7 is C E G B. C°7 is C E&flat;
G&flat; B&flat;&flat;. C half dim.
is C E&flat;
G&flat; B&flat;.
Note: they all contain C E G B something .
music.stackexchange.com/users/2808/Tim
I have never ever seen "o" denoting a major 7 chord.
A triangle can be used for maj7 in some books (e.g. the Aebersold play alongs), and 'o' is always diminished.
You may also encounter a slashed cirle (ø) for half diminished (min7 b5) chords.
music.stackexchange.com/users/1079/gurney alex
If you're playing from sheet music that's been handwritten, or perhaps photocopied multiple times, it's not uncommon for the small △ symbol, which denotes Maj7 to end up looking like small circle °, which, as you say, denotes Diminished.
Or if the copyist isn't particularly careful, the circle and triangle can end up looking similar.
music.stackexchange.com/users/9426/Brian THOMAS
o a degree sign for diminished triad or o7 for diminished seventh chords.
ø7 for half-diminished seventh chords. Δ7 a delta or triangle for major seventh chords.
Other signs are used capital M for major or lower case m for minor, etc.
You may have been mistaking the delta for a diminished sign.
music.stackexchange.com/users/23919/Michael Curtis
I had always thought that this notation indicated a major seventh chord: [C°].
At least it seemed to make sense that way in some of the chord sheets I have used in the past.
This must have been an error.
As Cmaj7 is a static chord (or eventuell would lead to F)
it couldn't be substituted by C°, while latter might have a dominant function to Db, (or E, G and Bb) and certainly can't be replaced by Cmaj7.
Edit: As the other answers confirm C°7 was never meaning C major 7 .
` I've edited the last words ...
was never a major 7 as someone could understand A-major 7 ( a was meant as article not as chord).
music.stackexchange.com/users/56219/Albrecht Hügli
While answers are correct that C° means C diminished and CΔ means CMaj7, there is more to the story here.
In jazz, C° is a very common delayed resolution to CMaj7.
For example, in a simple ii-V-I: | Dmin | G7 | CMaj7 | CMaj7 |
it's very common to hear pianists harmonize this with:
| Dmin | G7 |
C°7 CMaj7 | CMaj7 |
In this context, the scale typically used over C°7 is C whole-half (enharmonically, C D E♭ F G♭ A
♭ A♮ B ).
This diminished scale contains the BMaj triad, which resolves up a half step to a CMaj triad when we reach the CMaj7 chord.
However, while the C°7 can be voiced with a BMaj triad, this isn't necessarily and isn't always done in jazz.
We can simply voice the C°7 chord in its typical fashion.
This isn't all that different from a common-tone diminished 7th chord , which resolves ii o7 to IMaj (for example, D♯ o7 to CMaj).
After all, D♯ o7 has the same notes as C o7 .
But in a jazz context, the bass is often still playing the I, establishing the progress
as i o7 - IMaj7.
Lead sheets may show this i o7 chord written into the bar, or they may show it above the bar in parentheses to indicate that it's a possible chord substitution.
Or, it may not be shown at all, in which case the musician(s) are choosing to add this substitution into the music on the fly.
music.stackexchange.com/users/40842/jdjazz
I had always thought that this notation indicated a major seventh chord: [C°]. At least it seemed to make sense that way in some of the chord sheets I have used in the past.  I am now finding that it more commonly seems to represent a diminished chord, as in [Ab°(b13)] or [A flat dim flat 13]. My first question is has anyone here ever seen [°] used to represent a major seventh, or am I labouring under an illusion ;-)
Does C° also indicate a major seventh chord?
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music.stackexchange.com/questions/94021
music.stackexchange.com/users/65585/Al Dunbar
 C° is always C diminished. C major seventh can be signed with a triangle after the C.You may also encounter a slashed cirle (ø) for half diminished (min7 b5) chords. If you're playing from sheet music that's been handwritten, or perhaps photocopied multiple times, it's not uncommon for the small △ symbol, which denotes Maj7 to end up looking like small circle °, which, as you say, denotes Diminished.   C° is always C diminished
C° is always C diminished. C major seventh can be signed with a triangle after the C. Half-diminished is signified by a circle with a diagonal line through it. Lead sheets may show this i o7  chord written into the bar or they may show it above the bar in parentheses to indicate that it's a possible chord substitution. In jazz, C° is a very common delayed resolution to CMaj7. It's not uncommon for the small △ symbol, which denotes Maj7 to end up looking like small circle °.