it's not an arabic so it's better to keep the way that it's but if you want a translation we can say
"مجموعة إعلامية متعددة النشاط"
OR
"شركة اعلامية متعددة الوسائط"
OR
"مجموعة إعلامية متكاملة"
and I will give you advice, if you want to ask a question ,you have to explain properly what you mean so people can help you and gives the right answer,I didn't know what"omnimedia" means so I had so google it to come up with this translation