Yes it is possible, but like I said not recommended cus can get confusing.
A girl on another site thought she was pg cus she tested using an OPK and got a test line although it wasn't as dark or darker than the control...she thought it was a + therefore thought she was pg (she was at the end of her cycle) when af came a day later she was gutted.
If you wee on one, and get a + and you're at the end of your cycle, it's very possible you're pg.