Can someone tell me how safe is it to take the BART from San Francisco to Oakland airport in the evening? Around 7 pm.

At 7pm, you should still be fine. Just do what you would do in any big city--try to stay around others, be aware of your surroundings, etc.

Why do you have the impression BART is unsafe?
There should be plenty of folks taking the same trip around that time.
Where would you be getting on in SF?

completely safe. i take it all the time and at that time as the other responses said there are lots of people taking bart. note to get to the oakland airport you will have to take a shuttle from the bart station to the actual airport...fairly painless, it costs two dollars...exact change or on your bart ticket.

If we fly in Oakland, we always take BART to get in town. I wish we could do that from the airport. But, it is totally safe, and very convenient.

BART is a very safe means of transportation in the Bay Area. Aside from the weirdo on a 11:30pm train, or the odd bickering between two people, your biggest problem with bart will usually be the fares (but not the airport fare cause it is a deal) and the dirty seats.

Just watch your belongings and you will be fine!