Is OO gauge compatible with HO gauge?

There is a slight difference is size but both run on the same width track (16.5mm).

As for the scale, OO Scale is 1/76th which is 4mm per foot, whereas HO Gauge is 1/87th that is 3.5mm per foot.

For scenery, the issue is non existent as plants do grow in various sizes and shapes and there is no "correct" size for a tree or a tomato plant.

HO gauge figures may appear slightly smaller but we cannot all be six footers, can we!

Regarding building, the same apply: HO buildings will be smaller however the issue is more likely to be in the style. OO buildings will depict UK buildings while HO ones are going to be European or American. For a shed or a greenhouse, it does not matter however for a house or a pub the looks will be totally different, regardless of scale.

So, in conclusion, yes, OO gauge and HO gauge are compatible however you need to use your judgement to avoid obvious mistakes: an American pub in the middle of a Welsh country village is likely to look a bit odd.