I think, "I gave my seat to an old woman in the bus." 'In' to indicate you are giving your seat inside the bus, whilst 'on' makes me think that you gave your seat to her before riding the bus.

It's correct. It's a little more common to say 'on the bus', but both are acceptable.

Thanks to everyone

I made room for an old woman on the bus. Just change the in to on, otherwise perfect!