A brace from Charlie MacDonald and a goal from Kevin Lisbie were enough to see off the promotion hopefuls, who replied through Brett Pitman.
The Cherries are experiencing a worrying loss of form having slumped to their fourth straight defeat.
Howe said: "The goals that we conceded were really poor.
"At any level that you play, you can't gift teams goals and I felt we did, particularly with the first two goals.
"Leyton Orient didn't have to work hard for them, we gift-wrapped them and unfortunately, that's something that has crept into our game in the past few weeks. It's something we need to address immediately."
The Cherries were without injured central defender Tommy Elphick, and Howe added: "I don't want to talk about players who weren't playing but Tommy is our leader of the team and he's been a voice as well.
"Take his voice out of the group and it's made a difference to be honest. I've felt we've missed his influence, his demeanour, his leadership and his voice. But he's not here and we've got to get on with it.
"It's still not an excuse because the players that are playing are good enough, they've proved that. Teams lose players at key times, that happens.
"We scored a great goal to get back in the game at 2-1 and we were the team in the ascendancy for long periods after that and we'd had chances in the first half as well. But once the third goal went in, it was really difficult."