Character with codepoint #o223 is displayed as \223, do I have a font problem?

[N. Jackson]

I'm finding that the name "Oscar" (with an accent on the initial letter)
is displayed as

    Óscar

That first character (the accented A) is:

                 position: 258 of 309 (83%), column: 41
                character: Ã (displayed as Ã) (codepoint 195, #o303, #xc3)
        preferred charset: unicode (Unicode (ISO10646))
    code point in charset: 0xC3
                   script: latin
                   syntax: w    which means: word
                 category: .:Base, L:Left-to-right (strong), j:Japanese, 
l:Latin, v:Viet
                 to input: type "C-x 8 RET c3" or "C-x 8 RET LATIN CAPITAL 
LETTER A WITH TILDE"
              buffer code: #xC3 #x83
                file code: #xC3 #x83 (encoded by coding system utf-8-unix)
                  display: by this font (glyph code)
        xft:-PfEd-DejaVu Sans 
Mono-normal-normal-normal-*-12-*-*-*-m-0-iso10646-1 (#x85)

    Character code properties: customize what to show
      name: LATIN CAPITAL LETTER A WITH TILDE
      old-name: LATIN CAPITAL LETTER A TILDE
      general-category: Lu (Letter, Uppercase)
      decomposition: (65 771) ('A' '̃')

    There are text properties here:
      fontified            t

and the second character is:

                 position: 259 of 309 (83%), column: 42
                character: “ (displayed as “) (codepoint 147, #o223, #x93)
        preferred charset: unicode (Unicode (ISO10646))
    code point in charset: 0x93
                   syntax: w    which means: word
                 category: l:Latin
                 to input: type "C-x 8 RET 93" or "C-x 8 RET SET TRANSMIT STATE"
              buffer code: #xC2 #x93
                file code: #xC2 #x93 (encoded by coding system utf-8-unix)
                  display: by this font (glyph code)
        xft:-PfEd-Unifont-normal-normal-normal-*-12-*-*-*-d-0-iso10646-1 (#x96)

    Character code properties: customize what to show
      old-name: SET TRANSMIT STATE
      general-category: Cc (Other, Control)
      decomposition: (147) ('“')

    There are text properties here:
      fontified            t

Does this mean that the character with codepoint #o223 is missing from
DejaVu Sans Mono (my default font), or that something else is wrong? (I
tried setting my default font to several other faces but didn't see any
change.)

I'm guessing it's some sort of composition problem, which takes me a
long way beyond anything I know anything about!

Thanks in advance for any pointers,
N.