Yes he is a powerful false prophet. His basic theology is Protestant but …
More often than not, Osteen sounds like an inspirational life-coach, instead of a herald of the gospel. He often preaches about how people can improve their lives, be prosperous, and experience happiness. Noticeably absent in Osteen’s optimistic message is any mention of sin or repentance. The atonement of Christ provides us with healing and the abundant life, according to Osteen, but apparently receiving forgiveness from a holy God is not necessary.
Overarching all that Joel and Victoria Osteen teach is the prosperity gospel: God’s will is that we be blessed with material things, a view that contradicts 1 Timothy 6:6. As the Lakewood Church’s website says, “To be successful in your walk with God, commit to honor God with your finances. When you commit to give the Lord the first 10% of your income, God promises He will pour out blessings you cannot contain. Tithing is the first key to financial prosperity.” Lakewood’s website then quotes the favorite proof-text for this teaching, Malachi 3:10. It is a prime example of taking Old Covenant promises to Israel out of context to apply them to New Covenant believers.
Another problem in Osteen’s message is his promotion of name-it-claim-it or word-faith theology: “We have to conceive it on the inside before we’re ever going to receive it on the outside,” Osteen writes. “If you don’t think you can have something good, then you never will. The barrier is in your mind. . . . Your own wrong thinking can keep you from God’s best. . . . To experience [God’s] immeasurable favor, you must rid yourself of that small-minded thinking and start expecting God’s blessings, start anticipating promotion and supernatural increase. You must conceive it in your heart before you can receive it. In other words, you must make increase in your own thinking, then God will bring those things to pass” (from Your Best Life Now, chapter 1). There is nothing biblical about such teaching. There is no power inherent in positive thinking, and we do not create our own realities. God is not our servant, standing by and waiting for us to fire up our imaginations so He can lavish us with material goods. Jesus told His followers to “give up everything you have” (Luke 14:33), not to seek to get more.
https://www.gotquestions.org/Joel-Osteen.html
Yes he is a false prophet plugging the prosperity gospel which runs counter to biblical teachings.
The prosperity gospel: Why Joel Osteen believes that prayer can make you rich
The long, strange history of a quintessentially American theology.
Joel Osteen and his Lakewood Church, one of the largest megachurches in the country.
The prosperity gospel has its roots in an American occult tradition called New Thought
Today’s prosperity gospel was also shaped by pro-capitalist and Pentecostal thought traditions
Many of the evangelical leaders that surrounded Trump are proponents of the prosperity gospel.
https://www.vox.com/platform/amp/identities/2017/9/1/15951874/prosperity-gospel-explained-why-joel-osteen-believes-prayer-can-make-you-rich-trump