[ealgeone]

If He is the same in both OT and NT, then who are the muslims worshipping??

He is not Allah.

[af_vet_1981]

If He is the same in both OT and NT, then who are the muslims worshipping??
He is not Allah.

Your argument is illogical.
1. Allah is found in both the OT and NT. There have been Arabic Bibled much longer than there have been English Bibles. The very first verse in Genesis has Allah.
2. Aramaic and Arabic speaking Christians have professed to worship and adore the God of Abraham, Isaac, and Jacob for almost two millennia.
3. Hebrew => Eloah/Elohim with El used in compound constructs
4. Aramaic => Elaha
5. Syriac => Alaha
6. Arabic => Allah
7. if you make the issue of Islam's error in rejecting Jesus as the Son of God and God the Son as your criteria, what do you do with the Jews who do the same, and do not even regard Jesus as a holy prophet ? Both Jews and Moslems profess to worship and adore the God of Abraham, Isaac, and Jacob in their own respective languages, as do you.