I use Chestnut food safe oil which I think is based on liquid paraffin with added dryers, or just plain liquid paraffin (mineral oil). Some food oils are okay too, but some stay sticky and attract dirt and dust, some go rancid, and I forget which are which !
I used liquid paraffin on mine.
Go to the chemists and ask for Liquid Paraffin BP, it is medical grade, great for cutting boards and cheap. I gave mine several coats and then finished with a mixture of Liquid Paraffin and Real Beeswax (just warm the two gently together).
You may get some funny looks if you order more than one 150ml bottle (which should be enough), as apparently it is used as a laxative!
I used the "once a day for a week, once a week for a month and once a month forever" routine and my maple and purpleheart cutting board still looks like new.
I would not use Omso Top Oil, I have just used it on an oak worktop and it doesn't smell very natural, certainly not for direct contact with food
I know that Osmo Top Oil is good for real wood kitchen work surfaces but i haven't heard much about it being used for chopping boards. It's says that it's made from 'natural' oils so shouldn't be a problem but there are other products out there which specify that they're safe for contact with food surfaces such as chopping boards.