  Optical images and spectra, both ground based and taken by Hubble Space
Telescope (HST), of the young, luminous O-rich supernova remnant in the
irregular galaxy NGC 4449 are presented. HST images of the remnant and its
local region were obtained with the ACS/WFC using filters F435W, F555W, F814W
(B, V, and I, respectively), F502N ([O III]), F658N (Halpha + [N II]), F660N
([N II]), and F550M (line-free continuum). These images show an unresolved
remnant (FWHM < 0.05 arcsec) located within a rich cluster of OB stars which
itself is enclosed by a nearly complete interstellar shell seen best in Halpha
+ [N II] emission approximately 8'' x 6'' (150 pc x 110 pc) in size. The
remnant and its associated OB cluster are isolated from two large nearby H II
regions. The ACS [O III] image shows the remnant may be partially surrounded by
a clumpy ring of emission approximately 1'' (~20 pc) in diameter. Recent
ground-based spectra of the remnant reveal (1) the emergence of broad,
blueshifted emission lines of [S II] 6716, 6731, [Ar III] 7136, and [Ca II]
7291, 7324 which were not observed in spectra taken in 1978 -- 1980; (2) faint
emission at 6540 -- 6605 A centered about Halpha and [N II] 6548, 6583 with an
expansion velocity of 500 +/- 100 km/s; and (3) excess emission around 4600 --
4700 A suggestive of a Wolf-Rayet population in the remnant's star cluster. We
use these new data to re-interpret the origin of the remnant's prolonged and
bright luminosity and propose that the remnant is strongly interacting with
dense, circumstellar wind loss material from a ~20 Msolar progenitor star.
