The opposite is to 'board a bus'. 'Board' and 'alight' are the formal versions of 'get on' and 'get off'. It's also worth noting that we never use the word 'alight' in everyday contexts. It's one of those words that you read in official notices in stations and hear in official announcements, but you never come across it in normal English. We just say 'get on' and 'get off'.

'alight' means to get off the bus, so you can say 'get on the bus', or 'board the bus' hope this helps

The opposite would be "board the bus." However, although native speakers use this we commonly say "I get( or got )on the bus. We then say "I get( or got) off the bus. Although " to alight from a bus" means the same you will not hear it commonly used.