Weather is probably a major factor when you plan your trip to Oeiras, and keep in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually October and September with an average temp of 86°F, while the coldest months are July and March with an average of 81°F. The rainiest months in Oeiras are March, February, January, and April, with each month seeing an average of 7 inches of rainfall.