Using the words Ibadah and Uluheeyah are interchangeable when speaking about Allah’s oneness in terms of worshipping him. But there is a slight difference that all must bear in mind and inshallah the students of Tawheed would enjoy this.
Ibadah refers to the actions of the worshipper himself. As it is a comprehensive name for everything which Allah loves and is pleased with.
Uluheeyah refers to the Sifah or description of Allah. Meaning that Allah is described as the one who is worthy and deserves all worship. So that even if the whole of mankind were to worship other than Allah he would still remain the only one deserving of all worship.
Hence Ibadah refers to the action of the slave whereas Uluheeyah refers to a permanent attribute of Allah ta’ala.