That Sigma is a very good lens. But 30mm is about what a standard 50mm was on your OM.
Thing is that for landcapes you need a wide-angle, and for animals a large tele. Don't go for lenses that can handle both as that is asking too much.

For a digital slr, a wide angle of 17-18 is ok.  As far as tele is concerned, unless you want to print at 12x8 inches, you can crop in the photo you took. Yet for animals (not macro!) a 200mm would be needed. As this is about 320mm in "normal" size, you need to shoot at 1/500 or shorter to get a sharp shot (Image stabuilisation can help a bit, but is far more expensive). Which is why it's best to get a lens with a wider aperture. Never go for a lens that has 5.6 or even less (higher number) as autofocus gets into problems!

Also, I guess a decent quality is enough. High-end stuff is superb, but the difference is only visible in the hands of very experienced people (not me!).

I can say from personal experience that the sigma 18-50 f:2.8 is a good lens.On the OM this would have been about 28-80mm. Good for landscapes up to portraits. I did not get the kit lens, but this one and it really fulfills what I expected from it.
If I ever want a real tele, it'll either be the 70-200 f 2.8 zoom (which costs more than I want to invest, unfortunately.) or the 150 f 2.8. Which is also quite expensive but the macro interests me a lot. Also, 150 is in fact 240 in "normal" size. This is a more than decent tele and , as said, it would allow me to crop about one third for viewing on screen, making it an impressive tele. The 2.8 allows to eventually add a teleconverter (taking it to 300 or nearly 500 in OM days) without losing minimal brightness. Also, it's difficult to take sharp shots with those lengths. (I even use a tripod on wide-angle, but that's me). You would either need a monopod or tripod. If you're not too critical, the 150mm can still be handheld. If you shoot at 1:250 or shorter that is.
Of course, tele's are expensive.

There are lots of good brands, so don't fix on one. Get the most for your money, but don't let yourself be fooled in "this-can-do-it-all". Because it often can't do anything good.

That is my opinion.  Feel free to follow your own intuition. That's what life's all about.

Happy rides!

There are no Borg. All resistance is fertile.