over 是介词或副词，不作为连词，你可能是把连词 once 写错了。
一、介词over作“在……上方”、“在……上”（有覆盖、遮蔽之意）
The sun shines over the earth.
二、介词over作“超过”、“在……上”
He has lived in Beijing over two years.
三、介词over作“通过”、“越过”、“溢出”
1.I heard the news over the radio.
四、介词over作“遍及”
1.The fog spreads over the town.
2.2.The mud was splashed over their clothes.
五、介词over作“对于”、“关于”（有支配、监督、保护、控制等含义）
1.There will be much discussion over this problem.
六、副词over作表语,作“完了”、“结束”
The test is over.
七、副词over作“多余”、“过度”、“太”
1.I gave you two glasses and had three glasses over.
2.How much is left over?
八、副词over作“越过”、“出”
1.Come over and see us next week.
2.He will go（或sail,fly）over to Japan.
3.The milk boiled over.
