You place the letter "o" before each mana symbol. Also, if you are searching for a string of mana symbols, you will need to get them in the right order. For example, if you wish to find each card which can make both

and

with a single activation, you will need to but them in the order

since the Oracle always orders them that way. The appropriate search string would then be something like:
add oWoB
Placing that in the search bar, I get Ancient Spring, Orzhov Basilica, and Orzhov Signet. By checking only the box labeled "Search mana cost," it is possible, in a somewhat cumbersome manner, to search for specific color combinations. Hope that helps.
Fun fact: for this reason, if you search for Oboro with the default boxes checked, in addition to the moonfolk-related cards from Saviors of Kamigawa, you will also find a number of other cards such as Channel the Suns and Xira Arien. This has been a known issue for years but they aren't making any move to fix it last I heard.