Let’s talk first about what an occupier is, and what can it and cannot do. Let’s also re-frame the question as to whether Israel is acting as an occupier in Palestine? Because it’s Israel, no one dares to take a position at the risk of being labeled “antisemitic”.
This makes any answer awkward and ambiguous, so let’s try to untangle what could be a just response to that question.
As most educated folks know, the Israeli-Palestinian conflict goes back to the late 19th century. In 1948, Israel after installing itself in the middle of the Palestinian territory declared its independence, and the following year, quite predictably, the Arab-Israeli War of 1948 broke out.
As a consequence of the war, Israel captured the West Bank, Gaza Strip, and East Jerusalem, territories that had previously been under the control of Jordan and Egypt. Following that conflict and as a military OCCUPANT, Israel never fully withdrew from these territories, and has continued to exercise control over them to varying degrees.
The Palestinian Authority, which was established in the 1990s, has some limited self-governing powers in the West Bank and Gaza Strip, but Israel still maintains ultimate control over security, borders, and other key aspects of Palestinian life. So it’s fair to say that under that post-war regime the Palestinians aren’t free and are under Israeli control.
Now, if we look how this status plays under international law treaties and conventions, we can see that the Fourth Geneva Convention, which applies to occupied territories, states that an occupying power must "take all measures in its power to restore and maintain, insofar as possible, the normal conditions of life of the civilian population." However, the convention also states that an occupying power is not required to grant self-government to the occupied population.
This said, the International Court of Justice (ICJ) has ruled that Israel's settlements in the occupied territories are illegal under international law. However, Israel has refused to comply with the ICJ's ruling, so Israel is making a mockery of International Law. Then we can still discuss if Israel is an occupier in Palestine. The Palestinians will argue that Israel is an illegally occupying power and that it has caused the displacement of millions of Palestinians, the destruction of Palestinian homes and infrastructure, and the denial of basic Palestinian rights.
On the flip side, Israel claims that it’s not an occupying power, but rather a sovereign power that has the right to control the territories captured in 1967 and that its presence in Palestine is justified for its own security, and that the Palestinians have repeatedly rejected offers of a two-state solution, which makes sense since the Israel initial settlement prior to statehood was never agreed upon by the Palestinians.
Further with a shrinking Palestinian area and its scattered nature, a two-state solution is no longer possible unless (and this is far from certain) we would return to pre-1967 boundaries. Still, and quite hypocritically, Israel is satisfied with a status quo that will enable it to gradually push the non Israeli disenfranchised population population out in an inexorable process of territorial infringement, lack of freedom, lethal violence and trickling down genocide.