It was disputed before the oil was found, but the ownership wasn't pushed much because the land was not particularly valuable.

In colonial times, first, both were owned by Spain. Then the Dutch colonized Guyana, but a definite border was never set between Guyana and Venezuela. Then the British conquered Guyana from the Dutch. The border dispute was inherited by Venezuela and Guyana.

It's not as simple as "Guyana has oil, Venezuela wants oil, Venezuela is invading Guyana for their oil." Instead, oil has been found mostly offshore of land Venezuela has always claimed as its own. On top of this, the oil is mostly being extracted by countries that officially consider Venezuela an enemy state, and those countries are deploying their military forces much closer to Venezuela's undisputed territory than their own (though this is, of course, nothing new to the United States, who don't even hesitate to conduct patrols in the South China Sea, let alone the South Caribbean).

It's impossible to say for sure, but if Guyana wasn't solidly aligning itself with the United States and its corporations, it's unlikely that tensions would be as high as they are.