You can flavour oil a few different ways, but my two favourites are:
Pour some oil in a pan and before you heat or cook anything, add some strong flavours like garlic, chilli or fresh rosemary. To do this cut your chilli in half, slice the garlic nice and thick or clean and dry well a few sprigs of rosemary. Add them to the oil, then turn on the heat to low. As the oil heats it will draw the flavours out, which will flavour whatever you throw in next. The trick is to throw out the garlic and chilli before they go brown as the garlic will get bitter and burn long before anything else gets cooked. This works well with any strong ingredients that have really strong flavours. You could even cool the oil, add some vinegar or lemon juice and use as a dressing!
Make it intense
Grab a clean (preferably sterilised) bottle with a screw top lid. The neck has to be wide enough so you can push your flavours into the bottle. Push through garlic, chilli, porcini mushrooms, rosemary, citrus peel, whole peppercorns - whatever flavour you want the oil to have. Add in the best quality olive oil you can afford - opt for an extra light virgin oil as the flavour is the most mild.* Close the bottle and place in your pantry for at least a week so the flavours draw out into the oil. You can now use this to cook or as a salad dressing.
* note - extra light virgin olive oil is extra light in flavour only, the fat or calorie count is exactly the same as any other olive oil.