Bash

without sed

f(){ printf %$1s|tr \  $2;}
c=${1//o/`f ${#1} B`
}
echo "${c//re/ `f $[${#1}-2] F` 
}"

87 bytes (thanks to @manatwork)

with sed

f(){ printf %$1s|tr \  $2;}
echo $1|sed "s/o/`f ${#1} B`\n/g;s/re/ `f $[${#1}-2] F` \n/g"

90 bytes

Bash, 87 bytes

Without sed:

f(){ printf %$1s|tr \  $2;}
c=${1//o/`f ${#1} B`
}
echo "${c//re/ `f $[${#1}-2] F` 
}"

Thanks to @manatwork.

With sed (90 bytes):

f(){ printf %$1s|tr \  $2;}
echo $1|sed "s/o/`f ${#1} B`\n/g;s/re/ `f $[${#1}-2] F` \n/g"

Bash

without sed

f(){ printf %$1s|tr ' ' "$2"; };c="${1//o/$(f ${#1} B)$'\n'}";echo "${c//re/ $(f $((${#1}-2)) F) $'\n'}"

105 bytes

with sed

f(){ printf %$1s|tr ' ' "$2"; };echo $1|sed "s/o/$(f ${#1} B)\n/g;s/re/ $(f $((${#1}-2)) F) \n/g"

98 bytes

Bash

without sed

f(){ printf %$1s|tr \  $2;}
c=${1//o/`f ${#1} B`
}
echo "${c//re/ `f $[${#1}-2] F`  
}"

87 bytes (thanks to @manatwork)

with sed

f(){ printf %$1s|tr \  $2;}
echo $1|sed "s/o/`f ${#1} B`\n/g;s/re/ `f $[${#1}-2] F` \n/g"

90 bytes

Bash

without sed

f(){ printf "%$1s\n"|tr ' ' "$2"; };c="${1//o/$(f ${#1} B)$'\n'}";echo "${c//re/ $(f $((${#1}-2)) F) $'\n'}"

109 bytes

with sed

f(){ printf "%$1s\n"|tr ' ' "$2"; };echo $1|sed "s/o/$(f ${#1} B)\n/g;s/re/ $(f $((${#1}-2)) F) \n/g"

102 bytes

Bash

without sed

f(){ printf %$1s|tr ' ' "$2"; };c="${1//o/$(f ${#1} B)$'\n'}";echo "${c//re/ $(f $((${#1}-2)) F) $'\n'}"

105 bytes

with sed

f(){ printf %$1s|tr ' ' "$2"; };echo $1|sed "s/o/$(f ${#1} B)\n/g;s/re/ $(f $((${#1}-2)) F) \n/g"

98 bytes