cute, pic. but with all due respect, that's just not how basic rules of english pronunciation work
it's a short o
rost + er = roster (not rō - ster)
fost + er = foster (not fō - ster)
nost + er = nostr (not nō - ster)
tostr ≠ toaster
rostr ≠ roaster
costr ≠ coaster
nostr ≠ noaster
etc.