Well it really could go either way, cap should be stronger and faster but oliver has also shown cosiderable speed, strength and reaction. He's thrown full grown men several meters across a room effortlessly but i'd say cap is stronger, one of his best speed or reaction feat was to dodge a bullet from near point blank range unaware, with a silencer, in the dark, during a storm(meaning he couldn't hear the bullet go off), and subsequently dissapear in the same move, si'd think arrow won't be outclassed in a fight. The fight would probably be decided by how quickly arrow could analyze Cap. I'm sure he can avoid the shield toss, and once he realizes how gravity defying the shield is, he could take it out of contention with a magnetic arrow. But then that's where it gets hairy. If Oliver can keep it a ranged fight he has a higher chance of winning, Cap is definitely able to catch his arrows, but he can work around that so won't exactly be easy. If it comes to CQC then cap has the advantage, MCU cap isn't as proficient in Fighting as arrow is, he didn't get much chance to learn during the war, and we don't know for sure how much he's learnt with shield, but we know Arrow has 10 years of constant fighting experience against everything under the sun, from tyrants to aliens, and has all sorts of martial arts training, even ninja training. so he could more than match Cap in skill but the problem would be cap having a significant strength and durability advantage.
So here's how I see it, it comes to a ranged battle Arrow wins 7 times out of 10, with mid to high diff, if it comes to a CQC, cap wins 8 times out of 10 in high to extreme diff. So judging from how they both would normally act, as soon as arrow sees how far cap is able to move and throw his shield, he'd try to put some distance, giving him a better chance of winning.But i'd be an interesting match up