omshell: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/omshell - from package: dhcp

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/omshell - from package: isc-dhcp-common

You may install the required package with command apt-get install {package}

Error in other languages:
omshell: bevel nie gevind nie
omshell: comandă negăsită
omshell: comando não encontrado
omshell: commande introuvable
omshell: command not found
omshell: käsku ei ole
omshell: không tìm thấy lệnh
omshell: komanda nerasta
omshell: Kommando nicht gefunden.
omshell: kommandot finns inte
omshell: komut yok
omshell: nie znaleziono polecenia
omshell: níor aimsíodh an t-ordú
omshell: no se encontró la orden
omshell: no s'ha trobat l'ordre
omshell: opdracht niet gevonden
omshell: parancs nem található
omshell: perintah tidak ditemukan
omshell: príkaz nenájdený
omshell: příkaz nenalezen
omshell: команда не найдена
omshell: командата не е открита
omshell: 命令找不到
omshell: コマンドが見つかりません