If my blood type is O positive and my bfs blood is O negative.Will it be hard for us to have a child

No, it doesn't effect it either way. But if U have blood issues or bad genetics, U shouldn't have kids.
But U & U'r gf can't donate blood or organs to each other & U'r kids (if U want to have kids) cause U'r blood types are too different, which would be dangerous. But U can donate it to someone else if they're blood type is close to Ur's.