We've heard that charge.
Naftali Bennet was subjected to it (and inadequately responded).
Before we start, let's not be afraid of words.
I am occupying my time writing this and you are occupying your time reading this. Both of us are probably occupying a chair while reading this. Your occupation maybe doctor, lawyer or bottle-washer.
First of all, actually there is nothing wrong or illegal in being a belligerent occupier. That's what America was after WW II. It simply means that one country who is administrating territory gained as a result of military action. In itself, it is not a value judgment but simply a description.
Secondly, that military action of Israel was defensive.
Thirdly, the territory held now by Israel was part of the original area set aside to become the "reconstituted Jewish national home". The right of Jews to that territory is certainly no less than Arabs and I would add, more so.
Fourthly, the Arabs of the Mandate area rejected the partition options twice, in 1937 and again in 1947, and in the latter case actually violated UN resolutions to stop their aggression, so Israel is not obliged to respect those dead letters.
Fifth, no Arab state of Palestine ever existed. No independent "Palestine" was conquered and occupied in 1967 (or in 1948 for that matter).
Sixth, even UN resolution 242 never mentions a "Palestinian people" or a state or whatever.
Seventh, the only reason no Jews resided in the presumed "occupied" territory of Judea, Samaria and formerly Gaza prior to 1967 was an ethnic cleansing campaign carried out violently by riots, pogroms, murders, rapes and other assorted means by local Arabs, many of whom emigrated to the area from other Middle Eastern countries (Jordan's Hashemite family only arrived in Transjordan, itself part of the original mandate area until 1922, from Saudi Arabia in November 1920), as well as Arab states during 1947-1949.
Eight, during 1948-67, the Arabs continued their terror campaign, first as fedayeen and then as the PLO, against Israel that did not possess Judea, Samaria and Gaza in acts of war and aggression thereby displaying a total inability to claim protection as a "conquered people".
Ninth, there are many more real illegal occupations existing that few pay attention to or treat with equal fervor or get that excited over as they do Israel's case.
Tenth, that ICJ judgment was not made on the issue of Israel's legality of possession but the justices simply accepted the presentation made to them. It was never defended directly. The "illegality" charge is baseless.
There are more points but ten is a nice figure and I'll stop here.