However, I was only given the option to put a ticket on it and then pick
up
this ticket from my local tube station. I didn't understand this, so rang
the helpline. I was told that if I want to use the Oystercard for bus
journeys, I have to top it up at a tube station or at a newsagent's.

That is basically the size of it. The credit has to get on your card
somehow. For those who regularly use a tube station they nominate that tube
station and when they pass through the gates on their normal journeys the
new ticket or credit gets added to the card.

I tend to top-up my pre-pay at credit card only touch screen machines at
tube stations when I occasionally pass through them. They tend not to be as
busy as other ticket machines.

I thought the whole point of the Oystercard was so that you could put
credit
on it online, in the comfort of your own home?

No, because you can't put credit or tickets on it online, you can only order
credit or tickets, you still have to collect it somehow (by passing through
gates normally as it happens). However in itself this is an extremely handy
feature of use to many users. Just not those who only ever use buses or
trams with their card.

G.