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.