The name of months, oceans, rivers, lakes and mountains are masculine.
There are words that do not follow the rule that word ending in 'o' is masculine. For example: avó - grandmother.
There are words that do not corrrespond to rule that word ending in 'a' is feminine.
Let's see some examples of the exceptions that words ending in 'a' may also be masculine. The following words are all masculine.

dia	day	chá	tea	sofa	sofa
mapa	map	é espião	spy.	problema	problem