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I have an Opus Spark 2 which has a Tiagra cassette 12-28 (10 speed). I would like to change for a 11-34. The cassette exists in, a guess, the same series (http://bike.shimano.com/content/sac-bike/en/home/components11/road/Tiagra4700/cs-hg500-10.html).
Is there something else I should check to be sure it is compatible? Will I have space problem with the derailleur and the jockey wheel?
You are sort-of correct in the assumption that there is a 11-34 cassette in the Tiagra range means it is compatible with a Tiagra rear derailleur, but it's actually only the long cage (GS) version that will take a cassette of that size.
Shimano has a nice set of pages that detail compatibility within a groupset, here's the Tiagra page.
The Opus web page for the Spark 2 shows it has a Tiagra 4700 series rear derailleur. It's not specified which cage length but the picture clearly shows a long cage (GS) version.
You can also check the RD-4700-GS spec page and see it is compatible with a 11-34 cassette.
You also need to check that the derailleur works with the FSA crank chainring sizes, which are 48-32 assuming you have a size S frame or larger.
Maximum front difference is 16 teeth - you are good there
Total capacity (the difference between the numbers of teeth on the largest front ring and rear sprocket, and the smallest front ring and rear sprocket) is 39 teeth - you need 39 (34+48-11-32), so again you are good.
If decide to fit the bigger cassette you will need a new chain as the required chain length will be longer (chains should be replaced when cassettes are changes anyway). You will probably also need to adjust the derailleur b-screw which controls how close the upper jockey wheel rides to the cassette sprockets.
