晚上到达希斯罗机场还能买到oyster卡并充值吗？

as long as the tube still runs-- and it is not on strike--- and it is before the London Underground closes all ticket offices (which is the main factor causing strikes)

有机器可以买卡和充值，除非地铁关门了。

只要地铁有就可以

7,8点应该么有问题

78点肯定没问题的