For your convenience, here are all the mappings available in oengsimple.vim.
" SIMPLE LETTERS
" uppercase
AE Æ capital letter ash (Unicode 0x00c6)
DH Ð capital letter eth (Unicode 0x00d0)
TH Þ capital letter thorn (Unicode 0x00de)
" lowercase
ae æ small letter ash (Unicode 0x00e6)
dh ð small letter eth (Unicode 0x00f0)
th þ small letter thorn (Unicode 0x00fe)
" prevent keymap of digraph
/A A (prevents AE -> Æ)
/D D (prevents DH -> Ð)
/T T (prevents TH -> Þ)
/a a (prevents ae -> æ)
/d d (prevents dh -> ð)
/t t (prevents th -> þ)
// / solidus (slash)
" LETTERS WITH MACRON
" uppercase
-A Ā capital a with macron (Unicode 0x0100)
-AE Ǣ capital ash with macron (Unicode 0x01e2)
-E Ē capital e with macron (Unicode 0x0112)
-I Ī capital i with macron (Unicode 0x012a)
-O Ō capital o with macron (Unicode 0x014c)
-U Ū capital u with macron (Unicode 0x016a)
-Y Ȳ capital wye with macron (Unicode 0x0232)
" lowercase
-a ā small a with macron (Unicode 0x0101)
-ae ǣ small ash with macron (Unicode 0x01e3)
-e ē small e with macron (Unicode 0x0113)
-i ī small i with macron (Unicode 0x012b)
-o ō small o with macron (Unicode 0x014d)
-u ū small u with macron (Unicode 0x016b)
-y ȳ small wye with macron (Unicode 0x0233)
-- - hyphen-minus (Unicode 0x002d)
/- - hyphen-minus (Unicode 0x002d)
" LETTERS WITH DOT ABOVE
" uppercase
.C Ċ capital cee with dot above (Unicode 0x010a)
.G Ġ capital gee with dot above (Unicode 0x0120)
.Y Ẏ capital wye with dot above (Unicode 0x1e8e)
" lowercase
.c ċ small cee with dot above (Unicode 0x010b)
.g ġ small gee with dot above (Unicode 0x0121)
.y ẏ small wye with dot above (Unicode 0x1e8f)
/. . full stop (period) (Unicode 0x002e)
""""""""""""""""""""""""""""""""""""""""
" OLD ORTHOGRAPHICS
" symbols not normally used in student texts but present in OE manuscripts
" http://www.wmich.edu/medieval/resources/IOE/mss.html
" http://junicode.sourceforge.net/
" INSULAR SCRIPT
" uppercase
#D Ꝺ capital Insular dee (D) (Unicode 0xa779)
#F Ꝼ capital Insular ef (F) (Unicode 0xa77b)
#G Ᵹ capital Insular gee (G) (Unicode 0xa77d)
#R Ꞃ capital Insular ar (R) (Unicode 0xa782)
#S Ꞅ capital Insular ess (S) (Unicode 0xa784)
#T Ꞇ capital Insular tee (T) (Unicode 0xa786)
" lowercase
#d ꝺ small Insular dee (d) (Unicode 0xa77a)
#f ꝼ small Insular ef (f) (Unicode 0xa77c)
#g ᵹ small Insular gee (g) (Unicode 0x1d79)
#r ꞃ small Insular ar (r) (Unicode 0xa783)
#s ꞅ small Insular ess (s) (Unicode 0xa785)
#t ꞇ small Insular tee (t) (Unicode 0xa787)
## # number sign (Unicode 0x0023)
/# # number sign (Unicode 0x0023)
" ABBREVIATIONS
*e ⁊ Tironian et (Unicode 0x204a)
-TH Ꝥ capital thorn with stroke (abbr. for Þæt) (Unicode 0xa764)
-th ꝥ small thorn with stroke (abbr. for þæt) (Unicode 0xa765)
*us a᷒ combining 'us' above (type after the letter) (Unicode 0x1dd2)
*_ a̅ combining overline (type after the letter) (Unicode 0x0305)
" PUNCTUATION
.. · interpunct (Unicode 0x00b7)
*. · interpunct (Unicode 0x00b7)
" MISCELLANEOUS
*i ı small dotless i (Unicode 0x0131)
*s ſ [https://en.wikipedia.org/wiki/Long_s long ess (s)] (Unicode 0x017f)
*W Ƿ capital wynn (Unicode 0x01f7)
*w ƿ small wynn (Unicode 0x01bf)
** * asterisk (Unicode 0x002a)
/* * asterisk (Unicode 0x002a)